Replace by fee. Zero-confirmation transactions have always been insecure; replace-by-fee is essentially Firesheep for this vulnerability -- it puts a friendly

Replace by fee. Zero-confirmation transactions have always been insecure; replace-by-fee is essentially Firesheep for this vulnerability -- it puts a friendly UI on it and drops the technical acumen required from "substantial -- you probably have to write your own complicated C++ code" to "you have to be willing to lie; that's it." (Replace by fee in a nutshell: All Bitcoin transactions are unstable for a period of time, generally believed to be "approximately 60 minutes." Exploiting this instability to retroactively invalidate a transaction was possible but beyond the ken of casual attackers. Replace by fee lets you reverse any transaction you made younger than N minutes by simply saying "I'm willing to outbid the fee I offered on that transaction. The new transaction does this new thing, for example replacing the $100 transaction 'pay from my left pocket to restaurant for dinner' into 'pay from my left pocket to my right pocket.'")