Old School (2003)

Revealing mistake: When the b.j. instructor gives Frank the black eye, Frank has a bandage on his neck where the dart will strike.

Revealing mistake: When Will Ferrell is drunk on stage talking about streaking and bends over, the students watching the screen are grossed out. The assumption is he is nude. However, when he starts running off the stage through the crowd you eventually see below his waist, and he is wearing underwear.

Gluck

Revealing mistake: When Frank shoots himself with the dart gun and he falls in the pool, you can see a weight belt through his blue robe. (00:53:50)

Revealing mistake: When Frank shoots himself with the trank gun you can see that he is wearing a prosthetic neck piece with the dart in it.

Revealing mistake: The shot of Luke Wilson rushing through a corridor of an unnamed airport to catch a plane (the name of the town/airport is never mentioned)- they show him rushing down a corridor of the "airport" which is actually a corridor in the LA Convention Center - the sign showing "South Hall," "Kenetia Hall", etc. is visible in the background.

Revealing mistake: When Frank and the Dean are falling down the stairs in the classroom, the stunt double for Frank has short brown hair with yellow highlights, which is quite noticeably not Frank's hair type.