Mistake: Not drawing through ellipses
This is something I'm pretty adamant about - you should be drawing through every single ellipse you draw for my lessons. That is, draw around the ellipse two or three times before lifting your pen. Two is ideal in my opinion, but three is also acceptable.
When you try to hit your ellipse in a single round, it's usually going to come out uneven and wobbly (due to drawing too slowly and carefully) or extremely loose (due to simply not having built up the muscle memory to nail an ellipse). Drawing through your ellipses gives your arm the chance to familiarize itself with what's being asked of it in that first pass, and then firm it up in the second.
Along with giving you an extra chance to build up the muscle memory, it also helps you maintain the confidence needed to achieve a smooth, even shape, without totally losing control.
As you get better, your ellipses will tighten up - the gaps between your successive passes will shrink and eventually your ellipses will appear much cleaner. At this point you'll probably be able to nail your ellipses in one pass, but I still want you to continue drawing through them for all of the Drawabox lessons. Outside of Drawabox, you're free to do what you like.