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.
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.
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.
Answer: Kel.