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: 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.
Diamonds Are For Roger - S1-E7
Audio problem: When Chris is hit by the old lady the sound is before the actions.
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.
Continuity mistake: Mo Sweater Blues: For the amount of water Kel splashes on Kenan the wet patch is way to big.
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.