LOL, I'm sure I've been guilty of a few of these that people have listed over my lifetime..
For me, yanking back and kicking at the same time. On what planet does that serve a purpose?
I guess it's not a riding habit necessarily, but using a harsher bit simply because it's easier, not because it's necessary.
Jumping with toes down. It doesn't bug me when someone's heels slip up a bit (mine do on occasion), but with toes blatantly pointed down... ugh. So unsafe.
No release whatsoever over jumps.
Unnecessary spur use.
Someone's butt slamming down on the horse's back at a sitting trot.
And most importantly... people who think they know everything in the saddle and refuse to listen to suggestions.