Continuity mistake: In the scene where Jan Brady is talking to Cindy and Peter in the bathroom, she is holding her contact lens. Then in the next shot she is wiping her face with a lemon, and in the very next shot the lemon has magically changed into a blue towel that she is wiping her face with.
Continuity mistake: When Greg is surfing he accidentally falls into the water, at least 50 metres from the shore. About 5 seconds later, Greg is lying perfectly on his back on the shore, unconscious. There is no way that he can he swim 50 metres in 5 seconds while being unconscious.
Continuity mistake: In the scene where Jan phones her 'boyfriend' George Glass, she has a yellow piece of paper with her which says 555-HUNK on it. When she is dialing on her rotary dial phone, at first the camera is on Jan, and the noise of her putting the first 4 numbers into the rotary phone does not match up to the movements of her arm. Then the camera closes in on the phone, and she dials 221, instead of 865 or 'UNK', for the last three letters of HUNK. Still, she gets the right place she was trying to call.