Deliberate mistake: During the movie, when Mulan is just a girl, you can see that her eyelashes extend beyond her eyes. When she is pretending to be a boy, her eyelashes are short (this makes perfect sense). But when she is in the medical tent her eyelashes are long even though she had no time to put on makeup. This is deliberate though, because without it she would have still looked like a boy, and just pulling her hair down wouldn't make her look any different (since that's how she looked in the pond). So they had to change her eyelashes to make her look more girlish.
Deliberate mistake: In the scene where Maya is looking for Rodney at the party and he scampers off into the bathroom, he miraculously climbs on top of a toilet in a matter of seconds (which isn't actually shown). There is no way a guinea pig could do this.