Continuity mistake: When Monica and Frank are driving in the car with Liam, the Crawford Power Plant can be seen just behind the car. At this point, they have crossed the Stevenson Expressway and they are heading south down Pulaski. However, in the scene where they stop the car and argue, as Monica drives away, the Crawford Power Plant can be seen on the left, right ahead of them.
Continuity mistake: When Frank and Liam are sitting at the bar in the Alibi, Frank orders 2 cokes. When Lip changes his mind about drinking a beer, he gets up to talk to Liam. The camera shows 2 cokes sitting on the bar. When the camera changes angles the cokes are gone. The camera changes back to the same angle and Veronica is handing them the cokes.
Continuity mistake: In the episode where Fiona is being harassed by Craig's wife, she's talking to Debbie about having a "birthday party." Debbie says her birthday is in October. Fast forward to season 7 when Monica returns and gets Franny back. She says she hasn't been that happy since the summer Debbie was born, to which Debbie replies that her birthday is in December.
Continuity mistake: When Debbie sits down on the couch with Carl and Liam, Liam is sitting on Carl's lap with Carl's left arm around him. When the camera angle changes, Liam is sitting next to Carl, and Carl's hands are in his lap. When the angle changes again, Carl's arm is once again around Liam. It changes back one more time to Liam sitting next to him before Carl gets up to answer the door.
Continuity mistake: During Frank's discussion with the candidate for mayor, there's a bus leaving the stop, it's not the same vehicle. In the first shot, it's 1717, with no advertisement or line number on the rear, while in the second it's 1691 with an ad above the licence plate and the number 82 on the display.
Continuity mistake: After Fiona's return back home from the club where she's worked as a waitress, the blackish strips on her cheeks turn from partially rubbed off (justified by her wiping the tears) to perfectly fine a moment later, then change several more times during her discussion with Lip.