Revealing mistake: When Cindy and Buddy get trapped inside the cold chamber, she starts rubbing him to a point that it becomes masturbation. But when she's sitting on his legs doing the thing, in one side shot you can clearly see she is not holding his privates and her hand is just going up and down, closed but empty.
Revealing mistake: When the couple split the pole, they get hit by a bus. As the shot cuts to a wide shot, and if you pause in the right place, you can see the couple have been replaced by dummies, just before the bus hits them. [This isn't a "mistake" -- this is just dissecting a movie, frame by frame, until you can spot the special effects.]