Just out of nowhere, an "error" I see animators make waaay too often is panning the camera in one direction while a person or object is racing in the opposite direction, so poorly timed that the person/object only appears for a single frame or two. (It's even in the OP for Star vs the Forces of Evil S3-4, when Kelly is struck down by the Goblin Dog goblin's shirt bazooka. An OP for fuck's sake.) Characters shouldn't move so fast in relation to the camera that you can't tell what they're doing.