Anime tracers.
You know the ones. They gloat about their artwork they can spent less than an hour on, but you look at it and it's just a blatant trace over of a screen shot from an anime, or just ripped from a page in a manga.
It infuriates me because you can never call them out on it, because non-artists can't pick up the obvious tell tale signs and just accuse you of being jealous. I think what really makes me mad though is that the people who do it think no one will ever be able to tell. They're so unskilled it never even occurs to them that skilled people can see an artwork that didn't have genuine skill behind it.
That and while I'm at it, people who trace over photos and pretend they're professional artists. It's just as obvious. WHY do it though. All that time you spent playing pretend could be spent actually becoming good. It's the biggest waste of time in the world.