Yes, there are mistakes. But a background character having weird eye direction is IMPOSSIBLE to mess up by accident. When you import a flash character into the scene it will have the same default pose/expression etc. every time. To change her eyes, you'd have to double click on her build symbol, double click into her head symbol, move one pupil, then NOT NOTICE IT FOR THE REST OF THE HOURS YOU SPEND ANIMATING IT.
Missing wings/ears are easily explained by the animator temporarily turning the symbol to a guide layer to see that the parts underneath are lining up, or by reusing stock animation of ponies running without wings, and forgetting to add in the wings.
Derpy is not an error, it's an animator who likely decided to make one of the background ponies look trashed at a party.