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.

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...

> 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.

The right mindset to understand the requirements of Bitcoin client design is, I think, that of someone trying to clone an online game client (e.g. Starcraft) in a way that doesn't experience network state desynchronization. You can do whatever you like in your client UI-wise, but the "rules engine" of your implementation absolutely must reduce an incoming event stream into the same world state as the reference implementation.

For this kind of problem, the easiest solution (when possible) is to extract the "rules engine" component of the reference implementation into a library, and then have every other client just use it (and maybe contribute to its development) instead of trying to write something bug-for-bug compatible with it.

When you think about it, web browser rendering has much the same implementation-consensus problems as Bitcoin. And what do we see there? Everyone sort of converging toward just using the "rules engine" known as WebKit.

That's the way a blockchain works though. If there is a bug in the client that 99% of people use then of course you'll have forking blockchains. And all of this matters less anyway, because clients can fallback to blockchain.info if there is a fork detected, and just warn the user that the client may have a bug.

> because clients can fallback to blockchain.info if there is a fork detected

Unfortunately bc.i's implementation/instance is not immune to bugs, forking, lying or collusion. I don't know why you would expect it to be.

The point is that it is a trusted enough party. If the client gets a suspicious block it can check against that party as a way of seeing if the client itself may have a bug.

The bitcoin client has a fallback to blockchain.info when it detects a fork? What? I'm not sure I understand what you're saying.

A client, not, The client.

Say you were writing a client in a different language or for another platform.

Saying I was writing a bitcoin client I would never use some 3rd party as a fallback. As soon as a blockchain fork is detected you stop all transactions and wait for it to be solved by the miners. No other way around it.

What a wonderful idea. /s

Although I understand it to an extent. The core of software wouldn't be the core if it wasn't always used.