Night at the Museum (2006)

Revealing mistake: When the T-rex launches the table away, a couple of papers on it don't move a single inch, revealing they're glued to the table.

Sacha

Revealing mistake: In the park after the bad guys have been defeated and everyone starts returning to the museum, watch the T-Rex closely. In the overhead shot, he seemingly "floats" right through the branches of the trees which he normally would be moving. (01:34:50)

manthabeat

Revealing mistake: Towards the beginning of the film when Ben Stiller looks at the photo at him and his son, you can see that the photo is a fake. Ben and his son cast a shadow on the shadow behind them.

Revealing mistake: Obvious fake snow when Cecil runs away and the cowboy shows up in the remote controlled car.

Sacha