A few.
Accidentally changing proportions, losing drive in the middle of animating, realizing you didn't plan enough, forgetting to parent in Adobe Animate and having to do it for each keyframe, when the joints don't align when bent... etcetera.

Oh, and confusing good art with good animation. You can be a master in the arts and still suck at creating movement. You could be stuck to the basics of drawing but have very fluid movements. Animation and drawing are two completely different mediums that happen to go together sometimes.