My favourite condition/action combo is:
[ * ][restart layout]
* = a blank condition
No crap, I've lost count of the amount of times I've had students put their hands up (or just yell across the room) that their game is frozen and won't respond.
This is the first thing I look for in their event sheet and sure enough, it's almost always the same thing. I guess they figure that having a blank condition means it won't trigger the action.
The other one they always mess up (despite me telling them many, many times) is creating a layout but not linking an event sheet to it.