When the policeman drives up to switchable Sam he is clearly in the driver's seat. Then he is suddenly in the passenger seat when he calls switchable over. In the next view he is then out of the car on the driver's side again.
When Switchblade Sam takes an apple away from Gunther with his switchblade, the hand is obviously a fake plastic hand rather than the little boy's, as it wobbles when the switchblade makes contact with the apple.
Switchblade Sam's (Christopher Lloyd) hair is obviously hair extensions. This is especially obvious when he is looking up at the tree house when Dennis and his friends are in it hammering nails. You can see a white line on his head of where they are attached.