> In case you think that's theoretical, a bug with integer overflow let an attacker creating 184 billion bitcoins out of thin air. To fix this, a new implementation was pushed out, which ended orphaning more than 8 hours of transactions. In other words, even after 8 hours you can't be certain that Bitcoin transactions are durable.
That was in 2010. Most of the users take certain level of risk, also different kinds of people have different risk appetite. If you really think this is still a relevant risk, then probably you shouldn't use cryptocurrencies at all. Stay with the safe and proven almighty dollar.
https://en.bitcoin.it/wiki/CVE-2010-5139
Another interesting case is when bitcoind 0.8 changed the underlying database from BerkeleyDB to LevelDB. Due to different database lock limits, bitcoind 0.7 couldn't process a block that bitcoind 0.8 could. For 6 hours, there was a fork where old clients were using a totally different blockchain than new clients, because of this single block they couldn't handle. This is what patio11 means, that you have to be bug-for-bug compatible with the reference implementation.
http://bitcoinmagazine.com/3668/bitcoin-network-shaken-by-bl...