It has had forks which have produced blocks which: (1) would not have been produced by any previous nodes in the network, and (2) forced the updated software to ignore the original majority chain. If you check the software, you'll see explicit checkpoints at which the client is told to ignore certain blocks (identified by their hash).

Breaking the ability of the network to follow a valid majority chain is breaking backward compatibility. It's disingenuous to claim otherwise simply because the forked version eventually ended up with the longest chain.