What you think about?
There are various methods to succeed at double spending that do not require the immense amount of resources that a 51% attack would require.
An individual user can attempt a race attack. Or a solo mining operator or a mining pool can try a Finney attack.
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http://en.bitcoin.it/wiki/Double-spendingBut there's nothing stopping the broadcasting of invalid transactions that attempt to double spend. There is a chance this happen accidentally (e.g., having two copies of a wallet and spending from the second copy before the blockchain has finished downloading where it didn't know the coins were already spent). More likely when these transactions occur it is from fraudulent attempts to double spend. There have even been legitimate attempts to optimize "coin management" by services like SatoshiDICE that have gone bad and spewed many invalid double spend transactions to the network.
