Revealing mistake: Star Is Peeved: When Kel is eating the sugar cubes it is clear he isn't eating them at all. (00:07:50)
Revealing mistake: Cold War: Kel misses hitting Roger when trying to wake him up.
Revealing mistake: Mo Sweater Blues: When Kenan and Kel are using the high pressure hose to clean the sweater, if you look closely you can see that the sweater is pulled off the wall.
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.
The Cold War - S1-E8
Revealing mistake: Cold War: The drill hole Kenan makes at the start is larger than the drill bit.
Revealing mistake: The orange soda cans are supposed to be full but when they fall they are obviously empty.
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.
Revealing mistake: The parrot is a very obvious stuffed toy.
Revealing mistake: When Kyra goes upstairs after kissing Kel in the opening scene, it's clear the stairs don't go any further as you can see her shadow on the wall climbing down backstage.
The Lottery - S2-E3
Revealing mistake: The pink container in Kel's bags is very obviously empty by the way he handles it. (00:13:25)
The Lottery - S2-E3
Revealing mistake: The goldfish Kel grabs is very obviously a fake. (00:18:50)
The Lottery - S2-E3
Revealing mistake: When Kenan hits Chris in the mouth with the apple, you can tell it was placed in his mouth because he managed to bite the apple, and take a chunk of it out. Normally an apple like that would smash your teeth.
Revealing mistake: Kel is in the Orange Rain and if you look at the road you can see it is just paper by the way it warps. (00:09:50)
Get The Kel Outta Here - S2-E8
Revealing mistake: When Mrs. Rockmore opens the door there is a painted house visible outside. (00:02:55)
Get The Kel Outta Here - S2-E8
Revealing mistake: When Kel throws the masseuse, it is obvious he lands on a crash mat due to his height when supposedly on the floor. (00:06:30)
Get The Kel Outta Here - S2-E8
Revealing mistake: Kel walks away the wrong way when told he is not allowed in the house ever again. (00:07:50)
Revealing mistake: After Kenan gets his head stuck in the bars it becomes obvious on multiple occasions while Kenan is moving around that the "bar" on his right is fairly flexible, presumably made out of rubber for the benefit of the actor.
Answer: Kel.