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: When Red is shooting at Crews and Crews forces him off the road, the driver's front tire blows on Red's truck. You see it happen and see the blown tire again when he goes off-road. A few seconds later, Red drives away as if there is no problem. (00:58:05)