How I'm Spending My Summer Vacation - S2-E17
Plot hole: When Kevin is upset about Paul leaving for summer and is just lounging around the house, he asks why they can't go on vacation. His mom says "Maybe next Summer." In the very next episode, which takes place in the same Summer, they are going on a "traditional" Summer vacation.
Scenes from a Barbecue - S3-E24
Plot hole: All the kids and Arnie are about to play a basketball game. Darlene and Chuck Jr. battle in Rock, Paper, Scissors to see who picks first. They both throw scissors, and then Chuck Jr. says he gets to pick first. They should have done it over, since that would be considered 'a tie', he really didn't win. (00:12:10)