Revealing mistake: Winnie gets angry at her parents and runs out of the house. As her father stands up and she runs past the camera it wobbles, shaking the screen a bit.
Revealing mistake: When Winnie and Tuck are out in the canoe, and are being spied on, look at the shot that shows the profile of the guy's eye looking through the binoculars. His mascara is clumping.
Revealing mistake: In quite a few scenes in the movie, Winnie appears to be mouthing the lines of others.
Answer: I looked over and over, he is not wearing a band-aid. My guess is, since he has bumpy fingers and it was dark, you just thought you saw a band-aid.