Many. I should know.
Well, the one I always made was confusing convex with concave. Convex is the one that sorta looks like an almond. When drawing ray diagrams, confusing these two will definitely cause your answer to be way off.
Here is the ray diagram for a convex lens and for a concave lens:
As you can see, they're very different, so it's important that you remember the difference between the two lenses.
Hope this helps :)