Doug's Birthday Present / Doug's Fan Club - S4-E5
Other mistake: Doug's Fan Club: When Doug and Skeeter walk into the cinema, Doug sees some empty seats, and they start running towards them. In the shot of them running past the movie screen, their heads are transparent. (00:17:38)
Doug's in the Money / Doug's Sister Act - S4-E9
Other mistake: Doug's Sister Act: When Doug imagines himself as Smash Adams rising out of the floor in a chair, the animation cel code is visible at the bottom right corner of the screen. (00:17:05)