The most common problem is excessive ambition - students biting off more than they can chew. We've all been there. Animation is complex and time-consuming and I sometimes think that if we were honest with ourselves about how long it takes we would never actually do it.
But it is vital to know what you're getting into before you try to do it, and hopefully avoid giving up in despair at the size of the task. It took me ages to figure out exactly what goes into making a short film (or indeed a long film), so I recorded a short video setting out the whole of the CG Production pipeline, from concept through to opening night.
It runs about fifteen minutes.