Bitcoin Forum

Just a guess - suppose that you want to convince a bitcoin client that the network confirmed a transaction that didn't occur - would it be possible to fulfill the client with loads of connections from fake network peers, specifically developed to broadcast and confirm such a fake transaction?

Sure.  All you need to do is prevent that node from connecting to any honest nodes, and feed it a few blocks at the current difficulty.

problems with this:

• you'll still need computing power to generate the fake blocks, albeit not as much as a 50% attack. however, if you don't have much computing power, the other party may be suspicious that blocks aren't coming in. Also, I think there's a mechanism in bitcoin-qt to detect this. (it will say something like the "transactions may not be up to date, please update your client")
• all clients will try to make 8 outgoing connections randomly. this means you can't just flood a node with incoming connections.
• even if there is 1 connection to a honest node, the plan will fail because the real network can deliver blocks faster than you