Deliberate mistake: When "Legs" lowers the Pez duck named Ducky down through the removed lightbulb hole, to ring the doorbell, the lightbulb housing is bigger than the hole but they bring it up through the hole to remove it. (01:03:55)
Deliberate mistake: At the end of the movie when Gerry launches the go-kart in the air, all the spectators are about 50 yards away yet they are looking straight up as if the go-kart is flying above their heads.
Deliberate mistake: The huge bruise on Tommy's face, from the board hitting him, disappears in less than a day.
Deliberate mistake: After our heroes jump from the plane.as they are landing their parachutes the each come to a soft standing landing. if you jump with a standard Dash 1 Bravo parachute (which none of them had) your landing is similar to jumping from 14 feet high with no chute: you hit like a rock and collapse (the Parachute Landing Fall or PLF) They were all using stunt parachutes of a type that was unavailable in 1968.
Deliberate mistake: The lake that the girls splash about in is very shallow, as they easily stand and play in it. Roberta does a head-first dive from a high tree, into it. There's no chance she wouldn't have hurt herself. (00:32:35)
Deliberate mistake: After James and Kat enter Whipstaff and get the power turned on, when Kat heads to the staircase to go search for a bedroom, we can see a few of the metal stair runner rods attached to the stairs to keep the carpet runner in place. However, when James falls at the top of that staircase and gets rolled up in the carpet runner like a sushi, there are zero stair runner rods. (00:21:30 - 00:31:50)
Deliberate mistake: In the Search for the Stars contest, at the end of the song their mouths have stopped singing as they go to bow but you continue to keep hearing them apparently sing a few seconds longer as it fades out. This is a joke referring to the original Brady Bunch episode where the kids sing the song on live television and the song similarly fades out after they've stopped singing.
Deliberate mistake: When the house is splitting in two, the board game should have fallen long before it actually does. (01:25:25)