Revealing mistake: When the tower of Orange Soda collapses you can see it falls in segments of cans.
Doing Things The Hemingway - S1-E3
Continuity mistake: When Kel jumps into the window cleaners' bench it's only a fraction down from the roof. no more than 6ft. Next shot with Kenan the bench is further down.
Continuity mistake: Cold War: At the very start it is clear that Kel does not spit out the mouthwash into his hat. This is confirmed by when Kenan throws it no liquid can be seen.
Continuity mistake: When the President is being crushed in the Oval Office, Kel opens the sliding door. The door stays open but when left alone Kenan and Kel go to look at it and it is now closed.
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.
Diamonds Are For Roger - S1-E7
Audio problem: When Chris is hit by the old lady the sound is before the actions.
Factual error: Diamond are for Roger: The ease with which Kenan open the safe door is unrealistic. I have worked in a bank, and those doors are not light. It wouldn't move that quickly, that easily.
The Contest - S3-E16
Continuity mistake: During the whole first scene at Rigby's the clock never changes.
Who Loves Orange Soda? - S2-E4
Factual error: The doctor, on his fist look at the x-ray, claims Kel's ribs are bruised. Without looking more carefully, he wouldn't be able to say that. (00:05:10)
The Raffle - S3-E3
Revealing mistake: The woman who wins the TV supposedly hits Kenan with it, but very obviously misses. (00:17:05)
The Raffle - S3-E3
Continuity mistake: Chris comes into the shop and the clock says 8:55. He states the draw will be in 20 minutes, however, at the draw the clock still says 8:55. (00:11:00 - 00:12:00)
The Lottery - S2-E3
Continuity mistake: The Lottery numbers change from the draw to when claiming.
The Lottery - S2-E3
Audio problem: When Kenan is being hit by the swordfish, the sound is out of time with the actions. (00:20:25)
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)
Continuity mistake: There is a 5 minute scene in Rigby's at the end but the clock doesn't move at all.
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: 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.
Other mistake: When Roger writes the note after Kel glues his mouth shut, Kenan reads back far more than Roger could realistically have written in the short amount of time he took to write it.
Revealing mistake: The graduation stage appears to be set in front of a woodland area. However, when Kel climbs out of the bulldozer after having demolished the stage, you can see that it's actually a backdrop as the rail holding it up is visible.
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.