Continuity mistake: When Dooley is saying "She still wants to dance," Clark is standing next to him and they are roughly the same height. When the shot changes, Clark is suddenly a few inches shorter than Dooley.
Deliberate mistake: Luanne gets pregnant at the end of season 10 and gives birth in this episode. However, despite the passing of several months being implied through various storylines in the interim, she does not have a pregnant belly until this episode. The rest of the time her stomach was as flat as it usually is.
Lucky's Wedding Suit - S11-E12
Plot hole: In the previous episode, Bill had completely bulked up and then suffered a ruptured rectum, which would sideline him for six months. He remarks that by the time six months pass, all his muscle growth would be gone. This episode shows him to be back to his regular physique, and given the fact that it was established earlier in the season that Luanne was pregnant, the fact that she has no visible baby bump (her belly is still completely flat) suggests six months have not passed, meaning Bill could not have reverted to his flabby physique so quickly.
Love Hurts and So Does Art - S3-E18
Plot hole: There's no reason Hank or Peggy can't call the deli owner and tell him not to sell any more food to Bobby once he contracts gout.
Suggested correction: Bobby has generally earned his parents' trust. He might get into trouble occasionally, as many pre-teen boys do, but not so much that Hank and Peggy need to cover all the bases. They believe that he stopped going to the deli. As Peggy tells Connie, "He is doing everything he can to get back on his feet in time for that dance."
Take Me out of the Ball Game - S3-E24
Factual error: Near the end of the episode, Dale Gribble saves Team Strickland by catching a home run ball, hit by Kurt Bevacqua, with his hat. This wins them the game. However, according to the MLB Rulebook, if a player catches a fly ball with their hat, the batter takes three bases, meaning the game would continue and Bevacqua would be at third base. (00:21:00)
Character mistake: When Peggy is meeting Lucky's sister at Luanne's baby shower, she mentions that she lives across the street. However, the baby shower is taking place at Peggy's house, as evidenced by the layout of the house and the wallpaper.
Suggested correction: Peggy does live right across the street from Luanne, which is what she is saying. Myrna knows she was at Peggy's house. Since Myrna never met Luanne, she might not know where Luanne and Lucky live.
