What Is And What Should Never Be - S1-E20
Plot hole: How in the world could Keith be hired as a teacher? Not only is he a college drop-out with no teacher training, Dan mentions just an episode or two earlier how Keith is incapable of using modern machinery (specifically saying how every high school graduate will be more qualified than Keith).
Plot hole: Whenever Gretchin calls Mikey up to the stand to explain what he saw, he says that he had followed Spinelli and Randall to the wooden fence, where he Saw Randall on his knees in front of Spinelli, begging for forgiveness, then Mikey began running to the other side to stop Spinelli from pummeling Randall with the "rock" but didn't make it in time. We then later find out that Spinelli had actually rescued Ms. Finster's cat out of a tree and Randall hit himself with the rock out of spite. But it is impossible that Spinelli could have given up on hurting Randall, climbed up a tree, rescued a cat, climbed down, handed the cat back to Ms. Finster, gotten praise from Ms. Finster, then have Randall attempt to pick up a big rock, then get a smaller one and hit himself with it in under 3 seconds (the duration of how long Mikey ran before he heard Randall's scream).