Just as not every anime can be a winner, not every animated image can, either! With all those still frames to deal with, you can bet some stinkers are going to slip past quality control. They always do.
In Japanese, horribly drawn anime stills are called "sakuga houkai" (作画崩壊). They're not necessarily only mistakes, but definitely are the images that aren't quite up to snuff. In short, they're the cruddy images that either seem out of place or should've been scrapped.
Every once in a while, threads will pop up on Japanese bulletin boards with people asking for these kinds of images. They pop up on Western anime sites, too. Here's a collection of images—many of which are old chestnuts to anime fans—of sakuga houkai that popped up recently on 2ch, Japan's largest bulletin board. There are also some classic screwups that appeared on MyAnimeList a few years back. Have a look.
Animation Errors [MyAnimeList via 翻訳こんにゃくお味噌味(仮)]