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.
The Raffle - S3-E3
Continuity mistake: When at Rigby's at the start, the clock is constantly changing time. (00:02:00)
The Raffle - S3-E3
Factual error: When the Rockmmores turn the new TV on, the resulting light is impossibly bright. (00:06:20)
The Raffle - S3-E3
Visible crew/equipment: When Kel is saying his balloon is handsome you can see the stagelights reflected. (00:07:20)
The Raffle - S3-E3
Factual error: The Raffle: When Kel breaks the TV, he falls in the box completely, but shouldn't be able to due to the TV being in there. (00:08:15)
The Raffle - S3-E3
Audio problem: When Kel breaks the TV, the sound is a very clear overdub to make it clear the TV is broken. (00:08:15)
The Raffle - S3-E3
Visible crew/equipment: When Kenan and Kel are in the back of Rigby's, some stage lights are reflected in the fridge. (00:08:45)
The Raffle - S3-E3
Continuity mistake: Chris comes into the shop and does not lock the door but has to unlock it for the customers to come in. (00:11:00)
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 Raffle - S3-E3
Revealing mistake: Kel does not drop anything in the tombola. (00:13:20)
The Raffle - S3-E3
Continuity mistake: When the woman is taking the TV, the doors on the shop go from open to shut between shots. (00:17:00)
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
Revealing mistake: When Kel is trying to stop the woman taking the TV, he grabs her leg. If you look closely, he hasn't got hold of it at all. (00:17:10)
Continuity mistake: When Kel initially hits the wall with the mixer whilst they're making a cake the picture falls down, however when he chases Kenan over to the same wall with it seconds later, the picture is back on the wall again. (00:14:35)
Continuity mistake: When Kenan and Kel are in the kitchen trying to make the birthday cake, there is a blackboard behind them. The writing on the board disappears and then appears again during the scene.
The Contest - S3-E16
Continuity mistake: During the whole first scene at Rigby's the clock never changes.
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.