What percentage of Bitcoin transactions never complete? Occasionally transactions will be broadcast to the network that will show in a 0/unconfirmed status in the Bitcoin client. It is also the case that from time to time for various reasons those transactions will never confirm. One known reason for such unconfirmed transactions is a double-spending attempt, though these are usually quickly resolved by the network. My question is twofold:


Is there any source of data or is there a known statistic stating what percent of transactions hit 0/unconfirmed and never progress beyond this state (i.e. are resolved as conflicts by the network).
Is there anything aside from a double-spending attempt that can produce a 0/unconfirmed transaction that does not confirm.


Please note that I am not referring to the "generated" transactions by which miners are paid, but rather a point-to-point transfer of bitcoins.
I have watched for reports of a transaction that appeared live on the net that never got into a block. It has never happened, and I don't think it's likely to happen. The only practical way for it to happen is for the network to be split due to a connectivity problem or for a miner in cahoots with the person attempting the double-spend attack to mine a conflicting transaction (not accepted by the network) into a block before the transaction itself gets into a block.

I agree that accepting 0/unconfirmed transactions is sufficient for point of sale applications. Someone is very unlikely to attempt a double-spend in person for a few thousand dollars or less and is even more unlikely to succeed. (Update: Due to the risk of the Finney attack, I withdraw this argument.)

I think your guess is only reasonable within limits though. For big transactions, companies will sue people who charge them back. They can do the same thing with Bitcoin transactions if they ask for ID first. For face-to-face transactions, I don't think confirmations matter for transactions that aren't massive.

It's not a big deal for online transactions for a different reason. If we're talking a small payment for access to a web site, is anyone really going to bother with an attack? And does anyone care? Here, the percentage risk argument works. And for big online stuff, delivery's usually not immediate anyway (think PayPal, eBay, Amazon).