During the drag race with Julie's hair on fire, in a wide shot from the front of the cars, Julie can be seen with her friend's sweater on her head before she's seen taking it off to put out the fire a few moments later.
The grease slick on the rollerskating rink goes from a thin splotch to a large puddle after Donna passes through it.
When Dude uses his hair to make a grease slick on the rollerskating rink, his hair actually wipes up part of what he's supposed to be leaving behind. This is because the scene was shot with David Arquette running his head over an already present grease puddle.