It's one thing to make a mistake in a live action film, where people can walk across the back of shots, actors can miss their cue, or safety precautions have to be put in place that might end up caught on camera. In an animated film, where you have control of literally everything that happens in every scene, it's totally unforgivable.
There's a howler of a mistake in Disney's most recent mega-hit "Frozen", and we're sorry Elsa, but we just can't let it go. In the scene where she's singing the famous song which seemed to be everywhere you went for about a year after the movie's release, Elsa moves and her hair passes straight through her shoulder. It doesn't move across it, it doesn't pass over it, it goes straight through. Someone actually sat in front of their computer and animated that. What were they thinking?