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.
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.