Rory vs Penn, maybe? BJ was hit or miss before that fight, had a draw with Fitch and a close fight with Diaz. After Rory beat the piss out of him he was a shell of himself and went 0-5.
I think the last time Penn was really 100% in a fight was for the Jon Fitch draw.
BJ got lucky that his fight against Carlos Condit, that was supposed to happen before the Rory bout, got scrapped...
That was pre-GSP Condit too, when he was a savage... hey, another fight that changed a career drastically; the Natural Born Killer went into the GSP fight as the future, the new striker too agressive to be stopped, great record 28-4 and half of these losses were 10 years ago in Pancrase...
Since losing to St-Pierre that night, he's been struggling, and was on a 5 losses streak at a time, he's lucky fans love him so he's stayed in the UFC for so long.
The Lawler fight could have gone either way, too.
To be fair, he also altered GSP's career, as it was
the fight that seriously had him considered retiring.
The Nick Diaz fight was for the $$$, and it had been in motion since years ago, he'd been cancelled twice already...
Then by that time he decided that after one more, it'd be over....