user1428: What does a double spend look like? Can anyone link to an example on blockchain.info?
Will invalid half of the double spend simply remain at 0-confirmations, or might it ever reach 1 or 2? Will the standard bitcoind client provide any type of flag for known invalid transactions?

user323: You won't be able to see a double-spend in the block chain, because it is invalid. You will see one of the transactions included in the chain though. Blockchain.info has a page showing the most recent double-spends.
Double spend is just two (or more) transactions claiming the same input, so they will look like an ordinary pair of transaction. One of them will get included in a block, the other will eventually be forgotten by the network. A conflicting transaction to the one included in a valid block can never become a part of another block (at least until the first block will become invalidated by a 51% attack).
The standard client will not flag any invalid transactions, but it will reject any transaction that conflicts with a known transaction. It will, however, accept a transaction that is a part of a block, even if it conflicts with a 0-confirmation transaction that you know of.

1428: <p>What does a double spend look like? Can anyone link to an example on <a href="http://blockchain.info/" rel="nofollow">blockchain.info</a>?</p>

<p>Will invalid half of the double spend simply remain at 0-confirmations, or might it ever reach 1 or 2? Will the standard bitcoind client provide any type of flag for known invalid transactions?</p>
 323: <p>You won't be able to see a double-spend in the block chain, because it is invalid. You will see one of the transactions included in the chain though. Blockchain.info has a page showing the <a href="http://blockchain.info/double-spends">most recent double-spends</a>.</p>

<p>Double spend is just two (or more) transactions claiming the same input, so they will look like an ordinary pair of transaction. One of them will get included in a block, the other will eventually be forgotten by the network. A conflicting transaction to the one included in a valid block can never become a part of another block (at least until the first block will become invalidated by a 51% attack).</p>

<p>The standard client will not flag any invalid transactions, but it will reject any transaction that conflicts with a known transaction. It will, however, accept a transaction that is a part of a block, even if it conflicts with a 0-confirmation transaction that you know of.</p>
