Oh the many things students do in class that make me chuckle or wince.
1. Never run back at the group coming behind you. They will kick you or get mad at you or, at the very least, get confused. If you finish the combination - or you forget it - just keep moving forward or downstage.
2. Always start and end your jumps with demi-plie. I cringe whenever I see students finish a jump with a straight knee. Ouch!
3. Similarly, always start and end your pirouettes with demi-plie. You must use one to get momentum going and end with one to finish smoothly, not like a mannequin.
4. Never bend your knee before you pique. That's a big fat N-O. Instead bend the knee of the leg you're pushing off.
5. Never do rond de jambe en l'air with your hand while you're doing it with your leg. I see this sometimes and it makes me laugh (although, honestly, I do it sometimes when I teach!).
6. Never look down at the ground, especially before a turn or a jump. If you must see the floor for whatever reason, just cast a quick glance down, don't actually drop your head. It changes your alignment on a turn and works against you for height in a jump.
Any others you can think of? Happy dancing!