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.
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)
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.
Character mistake: In the flashback of Mr. Mac Fist hitting Roger in the eye, the fist hits Roger in his right eye. Once the flashback ends and goes back to the present Keenan and Kel, Keenan says "My dad couldn't see out of his left eye for 2 weeks."
Answer: Kel.