I think I've posed this question before, but I was recently bombarded with examples again--

What really throws you out of a fic?

I was invested in a long fic so I didn't actually back button, though I contemplated it, when the sentence described the main character's eyes as being blue, when they are clearly brown in the movie. Brown and blue are usually difficult to confuse!

The story had a lot of other issues, but basic fact problems really are annoying.