Revealing mistake: When the shop teacher walks up to Millie at the start of the film and slams the book, as he begins to step in front of her table, you can see him subtly "step down" even though he's on an even floor. Due to technical limitations, there needed to be a raised surface on the floor for the camera, a few inches off the ground. It was painted out digitally, but you can still see the actor step off the platform, making it look like he's "stepping down" on even ground. (Confirmed in Blu-Ray commentary.)
Revealing mistake: Clearly not Vince Vaughn during the opening kills as the Butcher seems leaner than Vaughn.
Suggested correction: Seems leaner" is just far too vague to be a mistake, and I didn't see anything that made me immediately think he was "clearly not Vince Vaughn.
Revealing mistake: When Millie (in the Butcher's body) knocks out the Butcher (in Millie's body) with the mini-golf club, you can see the Butcher react to being hit a split-second before he/she's actually hit. Slow motion helps, but is not required.
Revealing mistake: When Millie jumps on her shop teacher's back, the teacher switches from the actor to a stunt double as they begin to tumble about. Noticeably, his hair suddenly gets much darker and has less gray in it. You can also get brief glimpses of his face, and it's obviously not the actor.
