Here's an explanation[0] of the issue. As a summary: Apparently they were using the hash to track transactions, and would re-send bitcoins if the transaction d

Here's an explanation[0] of the issue. As a summary: Apparently they were using the hash to track transactions, and would re-send bitcoins if the transaction didn't appear to go through. This is obviously a problem if the transaction did go through, but with a different hash. Basically, two transactions which effectively do the same thing can end up having a different hash. Because of this, one should not rely on looking for the hash in the blockchain to see whether a transaction has "gone through". This has been a {known issue, flaw} since 2011, but some BTC exchanges (like Mt. Gox) continued to rely on the transaction hash to check whether transactions had been committed to the blockchain. If a transaction hadn't appeared to have gone through, they re-sent bitcoins (under a new transaction). This is a problem if the transaction did indeed go through, but under a different hash, as it would cause the same action (transfer money from A to B) to show up in the blockchain twice. 0. http://www.righto.com/2014/02/bitcoin-transaction-malleability.html http://www.righto.com/2014/02/bitcoin-transaction-malleabili...