It is simply impossible for someone to broadcast a transaction that does not follow the network rules to the network and expect anyone to accept it...
... 51% attack doesn't involve anything invalid or the breaking of rules.
I am looking at
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power. Are you saying that
Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
is not possible?
Reversing transactions (with 51% hash rate) made from an attack has nothing to do with breaking consensus rules.
The attacker has to have control over those 51% hashing power while sending an transaction.
For every confirmation of a TX (he is trying to reverse) he has to mine one block additionally more than the honest miner (in a shorter amount of time).
An attacker still has to broadcast blocks which meet the consensus rules. He can't just break them. And thats what ranochigo meaned.
If blocks will be mined 'on top' of invalid blocks (those who don't meed these rules) this will create a fork with only one person participating, the attacker.