Here are a few things that I have seen, but please don't take this as the end all be all gospel of what to avoid. There are lots of different things that can learned incorrectly.

1) Failing to keep the plane straight during takeoff run.
2) Climbing out too steep.
3) Not maintaining a constant altitude while turning.
4) Turns that aren't smooth, too sharp, etc.
5) Forgetting that there is a left stick.
6) Not lining up with the runway during landing approach.
7) Using ailerons instead of rudder to make course corrections when landing.
8) Using the elevator to extend the landing when you realize you're short.
9) Failure to learn the signs that a plane is about to stall, and what to do to avoid it, or recover from it.

Ken