I remember a horrible lesson years ago, the instructor was fetched from his lunch at the bar...he proceeded to belch whisky fumes, hiccup and chain smoke (yes, in a C152) throughout the lesson.I shudder to think what could have happened, I was pre-solo and wouldn't have been able to do anything if he'd passed out, which seemed likely.
I suppose the right thing to do would have been to complain to the CFI, but he had staggered back to the bar with all his airline cronies and I was far too intimidated to speak to anyone else there about it.
He got fired soon after because someone else did have the guts to complain.
These days things like liability are better appreciated. If you think your instructor is pissed, just refuse to fly with them, and don't feel intimidated as I did.