Revealing mistake: The orange soda cans are supposed to be full but when they fall they are obviously empty.
Revealing mistake: The parrot is a very obvious stuffed toy.
The Cold War - S1-E8
Revealing mistake: Cold War: The drill hole Kenan makes at the start is larger than the drill bit.
Doing Things The Hemingway - S1-E3
Revealing mistake: Kel takes the VCR player without unplugging it but Roger was watching a video seconds before.
Doing Things The Hemingway - S1-E3
Revealing mistake: The clouds are very obviously a set-drop when on the top of the CS tower.
Revealing mistake: When the tower of Orange Soda collapses you can see it falls in segments of cans.
Revealing mistake: When Kenan has the spider on his back it is blatantly a fake one. When Kenan panics, the spider doesn't move.
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.