When a bottom reaches under the pillow to sneak a whiff of poppers. The smell of poppers in general is like Kryptonite for my dick.
When I remove my dick to reposition ourselves, even when sheathed in latex, and it's covered in diarrhea. Session. Over!
Excessive moaning, groaning, or screaming during sex. Yeah, I know the dick is good, but stop trying to act like your interpretation of a porn star - even THEY are acting.