user662: I sent some bitcoins to an address and then looked on blockexplorer and it was there in about 10 minutes, which is what one should expect.
I did another send to a previously unused address and it took hours for the network to acknowledge it.  
I contacted the help desk at Mt Gox and they said this happens fairly often.  My question:  Is this a known phenomena and if so what causes it?

user1878: Perhaps your transaction took longer because the Bitcoin Days Destroyed is different..
I've seen this metric described as as a technique to throttle spammy transactions, where older coins get faster treatment than the same amount of coins that were just transferred to a wallet.

user575: This is caused by various characteristics of the transaction (the details of which I do not know), which make the Bitcoin client consider it low priority and hence only includes it in a mined block if it is sufficiently old.
Note also that http://bitcoincharts.com/bitcoin/ has a list of known floating transactions, you don't need to wait for it to appear in Block Explorer.

user85: Four factors affect this:
1) The size of the transaction. Smaller transactions get a lower priority. Abnormally small transactions look like spam.
2) The transaction fee. Abnormally low transaction fees may mean the transaction isn't even relayed. Miners have an incentive to include transactions with higher fees.
3) Luck. Some miners don't include any transactions. Some don't include any transactions without fees, even if the fee would normally be zero. So it comes down to who mines the next block or blocks. Also, 10 minutes is just the average. You could create a transaction and then by sheer bad luck the next block isn't mined for 20 minutes.
4) Security. The client 'dribbles' out its own transactions rather than broadcasting them to try to conceal the origin.

662: <p>I sent some bitcoins to an address and then looked on blockexplorer and it was there in about 10 minutes, which is what one should expect.</p>

<p>I did another send to a previously unused address and it took hours for the network to acknowledge it.  </p>

<p>I contacted the help desk at Mt Gox and they said this happens fairly often.  My question:  Is this a known phenomena and if so what causes it?</p>
 1878: <p>Perhaps your transaction took longer because the <a href="https://bitcoin.stackexchange.com/q/845/1878">Bitcoin Days Destroyed</a> is different..</p>

<p>I've seen this metric described as as a technique to throttle spammy transactions, where older coins get faster treatment than the same amount of coins that were just transferred to a wallet.</p>
 575: <p>This is caused by various characteristics of the transaction (the details of which I do not know), which make the Bitcoin client consider it low priority and hence only includes it in a mined block if it is sufficiently old.</p>

<p>Note also that <a href="http://bitcoincharts.com/bitcoin/" rel="nofollow">http://bitcoincharts.com/bitcoin/</a> has a list of known floating transactions, you don't need to wait for it to appear in Block Explorer.</p>
 85: <p>Four factors affect this:</p>

<p>1) The size of the transaction. Smaller transactions get a lower priority. Abnormally small transactions look like spam.</p>

<p>2) The transaction fee. Abnormally low transaction fees may mean the transaction isn't even relayed. Miners have an incentive to include transactions with higher fees.</p>

<p>3) Luck. Some miners don't include any transactions. Some don't include any transactions without fees, even if the fee would normally be zero. So it comes down to who mines the next block or blocks. Also, 10 minutes is just the average. You could create a transaction and then by sheer bad luck the next block isn't mined for 20 minutes.</p>

<p>4) Security. The client 'dribbles' out its own transactions rather than broadcasting them to try to conceal the origin.</p>
