No Contest - S1-E18
Revealing mistake: The photographs and news items relating to the Boston Barmaid Contest have been clumsily pasted onto a real newspaper. The paper upon which they were printed is cleaner and whiter than the off-white newsprint and the photographs have much greater contrast.
Continuity mistake: When Sam tells everyone it's closing time, he asks Harry how he got into the bar whilst he is playing snooker. He replies and then places a white snooker ball two inches behind a blue ball. There is also a yellow ball about three inches to the right of the blue ball. In the next shot, the yellow ball has suddenly moved a foot away from the blue ball. The yellow ball returns to its original position three inches away from the blue ball in the next shot. (00:00:10)

Continuity mistake: When Carla comes to the bar with her shrink, the lights aren't reflected in the door as they usually are. The lights are suddenly reflected on the door a further two shots later. (00:18:00)

Sam's Women - S1-E2
Continuity mistake: Just after Norm says "This man knows his animal kingdom" he looks at his watch and there's nothing in the other hand. In the next shot, his hand is suddenly on his beer glass. (00:20:40)
What is... Cliff Clavin? - S8-E14
Continuity mistake: At the point Cliff is disputing the fact that he thought his answer was right, on the right-hand side you can see the woman player's, Agnes, score. It says $400. Then in the next shot it changes to $3300. Then in a little bit later it changes back to $400. (00:14:30)
Feelings... Whoa, Whoa, Whoa - S11-E9
Other mistake: At the beginning of the episode when John Hill comes downstairs he says he hasn't the bar since the fire. But he was in the bar in the previous episode.
Character mistake: When Sam and Diane are embracing and saying goodbye, Diane asks, "Sam?" and Sam replies, "Yeah, Shel." Shelley is the actor's real name. (00:17:00)
Suggested correction: He says "yeah, chum?" It's another word for pal or friend, which is in line with everything else he has been saying.
Continuity mistake: When Coach is talking to Rebecca for the first time he is suddenly drying a shot glass then it disappears.
Give Me a Ring Sometime - S1-E1
Continuity mistake: When Carla is reading out an order to Sam, there is someone visible to the left of screen who isn't there in the next shot. (00:20:40)
Give Me a Ring Sometime - S1-E1
Continuity mistake: When Diane starts laughing, Sam walks over to Carla and she looks at him. In the next shot, Carla is looking down at her notepad. (00:20:35)
Continuity mistake: When Sam walks back into the bar, Carla turns her head away from Sam after he says "I forgot my bubble gum". Her head is facing Sam again in the following shot. (00:11:40)

Give Me a Ring Sometime - S1-E1
Continuity mistake: When Diane says "Nothing" to Sam, there is a crease on her top and part of her necklace is resting on that crease. 2 shots later, the crease is gone and the necklace is hanging freely. (00:21:25)
Give Me a Ring Sometime - S1-E1
Continuity mistake: When Coach takes Norm out of the bar, Carla walks past them. In the next shot, Carla is suddenly seen walking out of shot at the other side of the room without any time to get there that fast. (00:16:50)