Will a stop order get triggered if the floor is hit and trading is halted?
During a circuit breaker, no trading occurs. These policies have been implemented to maintain exchange liquidity since the NYSE nearly went bankrupt during the 1987 crash because many members had become insolvent. If an order is filled before the halt, it will stand unless busted.  During the Flash Crash, many orders were busted. quantycuenta is right, if a halt is in place, then no trading will occur, simple as that. But in the practice of risk management it is a little different. Want to remind you that you are assuming that trading is halted immediately upon the drop in price. That doesn't always happen, so if there is any time between the actual price drop and halt of trading, then it is possible that your order will be filled, depending on how liquid your security is. Also not every security has circuit breakers in place and the exact requirements to trigger a breaker is not public information.  In some cases, trades are ordered to be rolled back (reversed) by the exchange but this is usually reserved for institutional traders who make some sort of mistake.  This article below mentions day traders who bought at or near the bottom of the May 6, 2010 flash crash. This was before circuit breakers but I think it's a good story for someone looking to understand the finer workings of the electronic market.  http://www.marketwatch.com/story/book-takes-a-look-inside-professional-day-traders-1339513989350
"EDIT: It was System Disruption or Malfunctions August 24, 2015 2:12 PM EDT   Pursuant to Rule 11890(b) NASDAQ, on its own motion, in conjunction   with BATS, and FINRA has determined to cancel all trades in security   Blackrock Capital Investment. (Nasdaq: BKCC) at or below $5.86 that   were executed in NASDAQ between 09:38:00 and 09:46:00 ET. This   decision cannot be appealed. NASDAQ will be canceling trades on the   participants behalf. A person on Reddit claimed that he was the buyer. He used Robinhood, a $0 commission broker and start-up.  The canceled trades are reflected on CTA/UTP and the current charts will differ from the one posted below.  It is an undesired effect of the 5-minute Trading Halt.  It is not ""within 1 hour of opening, BKCC traded between $0.97 and $9.5"".  Those trades only occurred for a few seconds on two occasions.  One possible reason is that when the trading halt ended, there was a lot of Market Order to sell accumulated.  Refer to the following chart, where each candle represents a 10 second period. As you can see, the low prices did not ""sustain"" for hours.   And the published halts." Limit books are managed by exchanges. If an order is not immediately filled, it is sent to the book.  From there, orders are generally executed on price-time-priority.  The one major exception is the precedence hide-not-slide orders have over earlier placed visible slidden limit orders since unslidden orders are treated like a modification/cancellation. To an exchange, a modification is the same as a cancellation since it charges no fees for placing or canceling orders, only for trades.  The timestamp is reset, and price-time-priority is applied in the same way, so if a modified order isn't immediately filled, it is sent back to the book to be filled in order of price-time-priority. On most exchanges, if you place a limit order to sell at 94.64, you will be executed before the market can trade at a higher price. However most stocks in the US trade across several exchanges and your broker won't place your limit order on all exchanges (otherwise you could be executed several times). The likeliest reason for wht happened to you is that your order was not on the market where those transactions were executed. Reviewing the ticks, there were only 8 transactions above your limit, all at 1:28:24, for a total 1,864 shares and all on the NYSE ARCA exchange. If your order was on a different exchange (NYSE for example) you would not have been executed. If your broker uses a smart routing system they would not have had time to route your order to ARCA in time for execution because the market traded lower straight after. Volume at each price on that day: As mentioned in the comments: According to the message from the exchange all trades at or below $5.86 that were executed in NASDAQ between 09:38:00 and 09:46:00 ET got canceled.  If the short doesn't fall into those criteria but the long does, though luck the long is invalid, the short is valid.  Traders that got the short end of the stick in the end, should contact their brokers and inquire about this situation. Depending on the terms of the broker the short might get canceled as well. If not, then it's up to the market. The trader can keep or close the short. IMO, what the person in question should have done is hold on to the trade and see what happens at the end of the trading day.  He should've realized something was wrong when the price went from 8.xx to 0.8x. If the stock has dropped from $10 to $2 and now is range trading between $2 and $3, and you were not able to sell your shares earlier, then I would no be holding on to them now. As soon as the price hit $3 sell them. After you have sold them and you noticed the stock still range trading one strategy you could apply is to go long after the price bounces off the $2 support placing a stop just below $2, then as the price moves up you trail your stop up with the price. As it starts getting close to $3 tighten your stop. If it keeps range trading and bounces off the resistance at $3 and you get stopped out, you can either go short and reverse the process or wait for it to bounce off the support at $2 again. One word of warning though, the longer a stock range trades, the bigger the outbreak out of the rage (either up or down) will be, that is the reason why you should first wait for confirmation that the price has bounced off support/resistance before opening a position, and secondly why you should use a stop loss to get you out instead of just selling when it hits $3, because if it breaks through $3 you can continue profiting as it moves up. Is this due to the delay?  Yes, but the delay is caused by your broker and its affiliates.  Trailing Stop Order is not exchange native, meaning that the broker is responsible for keeping track of whether the stop price has been reached, and the broker is responsible for sending the subsequent Market Order to the exchange.  For certain exchange, even Stop Order or Stop Limit Order is not exchange native.  Is it common to be so different? No, only in times of extreme volatility. It depends on how you place your stop order and the type of stop orders available from your broker. If you place a stop market order and the following day the stock opens below your stop your stock will be stopped out at or around the opening price, meaning you can potentially end up with quite a large gap. If you place a stop limit order, say you place your stop at $10.00 with a limit price of $9.90, and if the price opens below $9.90, say at $9.50, your limit sell order of $9.90 will be placed onto the market but it will not be executed until the price goes back up to $9.90 or above. The third option is to place a Guaranteed Stop Loss, and as specified you are guaranteed your stop price even if the price gaps down below your stop price. You will be paying an extra fee for the Guaranteed Stop Loss Order, and they are usually mainly available with CFD Brokers (so if you are in the USA you might be out of luck). I don't have all the answers. On a illiquid stock, such situations do arise and there are specific mechanisms used by exchanges to match the order. It is generally not advisable to use market order on illiquid stock. There are lots of different variations here. I guess this comes down to specifications for individual exchanges, but I'm wondering if there's a standard here or a way to approach it from basic rules that clears up all these situations. There are quite a few variations and different treatments. Market order that are placed when the market is closed or just around market opening are traded at Market Open price that each exchange has a formulae to calculate. In the process Market Buy are matched to Market Sell at the Exchange calculated price. Not all order get matched and there could be spill over's. These are then matched to limit orders. Is this determined based on which sell order came first, or based on which would result in the best deal for the incoming buyer? Generally Market orders have highest priority of execution. why not just use a conditional order (http://www.investopedia.com/university/intro-to-order-types/conditional-orders.asp)? Like a one triggers one order? an order like this lets you place a buy order for the stock and if its executed another order is automatically placed. you could choose to let your second order be a stop order. so here's a company that offers stuff like this as an ex. (https://www.tradeking.com/education/tools/one-triggers-other-order) "Yes there is, it is called a One-Cancels-the-Other Order (OCO). Investopedia defines a OCO order as: Definition of 'One-Cancels-the-Other Order - OCO' A pair of orders stipulating that if one order is executed, then the   other order is automatically canceled. A one-cancels-the-other order   (OCO) combines a stop order with a limit order on an automated trading   platform. When either the stop or limit level is reached and the order   executed, the other order will be automatically canceled. Seasoned   traders use OCO orders to mitigate risk. I use CMC Markets in Australia, and they allow free conditional and OCO orders either when initially placing a buy order or after already buying a stock. See the Place New Order box below:  Once you have selected a stock to buy, the number of shares you want to buy and at what price you can place up to 3 conditional orders. The first condition is a ""Place order if..."" conditional order. Here you can place a condition that your buy order will only be placed onto the market if that condition is met first.  Say the stock last traded at $9.80 and you only want to place your order the next day if the stock price moves above the current resistance at $10.00. So you would Place order if Price is at or above $10.00. So if the next day the price moves up to $10 or above your order will be placed onto the market. The next two conditional orders form part of the OCO Orders. The second condition is a ""Stop loss"" conditional order. Here you place the price you want to sell at if the price drops to or past your stop loss price. It will only be placed on to the market if your buy order gets traded. So if you wanted to place your stop loss at $9.00, you would type in 9.00 in the box after ""If at or below ?"" and select if you want a limit or market order. The third condition is a ""Take profit"" conditional order. This allows you to take profits if the stock reaches a certain price. Say you wanted to take profits at 30%, that is if the price reached $13.00. So you would type in 13.00 in the box after ""If at or above ?"" and again select if you want a limit or market order. Once you have bought the stock if the stop order gets triggered then the take profit order gets cancelled automatically. If on the other hand the take profit order gets triggered then the stop loss order gets cancelled automatically. These OCO conditional orders can be placed either at the time you enter your buy order or after you have already bought the stock, and they can be edited or deleted at any time. The broker you use may have a different process for entering conditional and OCO orders such as these." "This is called a Contingent Order and is set up so if one order is filled (in this case) the other order is cancelled.  It's a common desire that one would wish to have a stop-loss in place but also a targeted sell price for their in-the-money sell point.  Your broker will tell you all you need to know about how to enter this, if you explain you'd like to place a contingent order. (As Victor noted below, your specific order would be a ""One Cancels Other"" or ""OCO"") Great first question, welcome to Money,SE." Stop orders and stop limit orders typically do not execute during extended hours after the general market session has closed. Stop orders are market orders and market orders especially are not executed during extended hours. Although there are exceptions because a broker can say one thing and do another thing with the way order types are presented to customers vs what their programming actually does. The regulatory burden is a slap on the wrist, so you need to ask the broker what their practices are. Orders created during normal market hours do not execute in extended sessions, different orders would have to be made during the extended session. Your stop order should execute if the normal market hour price stays below your stop price. So a stop limit would actually be worse here, because a stop limit will create a limit order which may never get hit (since it is above the best bid best ask) This is to protect your position in specific highly volatile market conditions.  If the stock is free falling and you only have a stop order at $90, it's possible that this order could be filled at $50 or even less. The limit is to protect you from that, as there are certain very specific times where it's better to just hold the stock instead of taking a huge loss (ie when price is whipsawing). TD will only sell the stock for you if there's a buyer. There was a buyer, for at least one transaction of at least one stock at 96.66. But who said there were more? Obviously, the stocks later fell, i.e.: there were not that many buyers as there were sellers. What I'm saying is that once the stock passed/reached the limit, the order becomes an active order. But it doesn't become the only active order. It is added to the list, and to the bottom of that list. Obviously, in this case, there were not enough buyers to go through the whole list and get to your order, and since it was a limit order - it would only execute with the limit price you put. Once the price went down you got out of luck.  That said, there could of course be a possibility of a system failure. But given the story of the market behavior - it just looks like you miscalculated and lost on a bet. The strategy could conceivably work if you had sufficient quantity of shares to fill all of the outstanding buy orders and fill your lower buy orders.  But in this case you are forcing the market down by selling and reinforcing the notion that there is a sell off by filling ever lower buy orders.  There is the potential to trigger some stop loss orders if you can pressure it low enough.  There is a lot of risk here that someone sees what you are doing and decides to jump in and buy forcing the price back up.  Could this work sure.  But it is very risky and if you fail to create the panic selling then you risk losing big.  I also suspect that this would violate SEC Rules and several laws.  And if the price drops too far then trading on the stock would be halted and is likely to return at the appropriate price. Bottom line I can not see a scenario where you do not trigger the stop, net a profit and end up with as many or more stock that you had in the first place.