This is not true, and I can say that because I work on Geth. We take great care to ensure that all clients behave according to the specification, and Geth has had similar faults in the past.
Surely it doesn't matter what the specification says? If the majority of the hashpower is using an implementation which deviates from the spec, then the blockchain will follow that code and not the 'correct' version. By the time the devs have fixed the bugs, it will be too late and too costly to roll back the chain and reverse all the subsequent transactions.
In theory you're right, yes. If the majority of the hashpower runs a specific version with specific consensus rules, then that version of the chain would "win".
But in reality, developers and others write the specification, which gets implemented in the clients and when a new version is available, the miners usually upgrade to the new version without any qualms what so ever. So in practice, the specification is what controls the network, as developers writing the clients implement things from the specification.
I mean in terms of bugs, rather than a 'rogue' client, i.e. not a deliberate attempt to cause a chain divergence.
If a 'broken' transaction gets accepted by a buggy client, and that buggy client has a majority of hash power, then that transaction is, by definition, not actually broken at all and everyone will be forced to accept it (because it will take too long to write a bug fix and re-write the blockchain history)