A bitcoin node tries to determine what the consensus is about the bitcoin ledger by looking for the longest block chain. However, in order to do that it must f

A bitcoin node tries to determine what the consensus is about the bitcoin ledger by looking for the longest block chain. However, in order to do that it must first collect blocks from other nodes and reject any invalid ones. If a certain implementation disagrees with the rest of the network about the validity of a block, then it will disagree with the rest of the network about the state of the ledger. This can then be exploited by an attacker, e.g. by paying a merchant with a transaction that seems to OK to him but is not accepted by the rest of the network. I don't think that this is a big problem though. A node knows that it is no longer in agreement with the majority of the network when it sees an "invalid" block chain that has the most proof of work done on it. In this case it should shut itself down.