For my team, it was a motor that decided to not break
completely, but not work at "regular" speed. So the advice of the painters tape to check to see if a robot is running straight is a great tip! For us, there was no way to correct for it because it seemed to not have a consistent slowing down in that motor. After discovering it, the kids had to rebuild the robot to make the 3rd motor the right wheel motor and move that "broken" motor up to the 3rd motor position where it's slower speed didn't affect the driving of the robot. I have since outlawed hand-spinning motors- you know when the kids stand there with the robot and just rip away at the wheels to make them move? Yeah. Ask them not to do that.
Our biggest problem by far was user error, but battery power really hurts you if you have programmed any timed missions, vs. degrees/rotations. Not that you want to redo any of your missions this close so game time, but you
might have time to update one section on the most critical missions if you used the timed distance option.
Good luck! And have fun :)