This slipup is pretty obvious and very noticable. About 10 or 15 minutes into the movie Charlie is sitting in the school dining hall, Amy sits down and starts to appologise to him for the argument they had the other night, the news comes on the TV and Charlie goes to watch it ignoring Amy. Amy is (understandably) annoyed about this and grabs someones sloppy Joe burger and rubs in in Charlie's face.

There is a close up of it and you can see Charlie's face covered in the burger and other stuff, a big slice of tomato on his right cheek slides down and falls off his face, you can still see his face in close up which now has only burger and mayo stuff left on it with no other tomato slices.

There is then a side shot of his face and low and behold the big slice of tomato is back on Charlie's face right where it was before, as Evil Ed is laughing and talking the slice again slides off Charlie's face and we get another close up where there is clearly no more tomato on his face, again it goes to the side shot and the tomato slice is back again!