Plot hole: At closing time, Cliff, Norm and Woody spontaneously decide to go to Cliff's house. Sam then enters the bar after his date at Melville's. Later in the scene, Rebecca asks Sam about the 'Three Stooges', to which Sam sarcastically replies that they went to Cliff's house. But how could he possibly know that since he entered the scene after they had already left.
Other mistake: Right after Sam tells Carla he will sleep with the next woman who walks through the door, in the background there is an "Extra" (blonde-haired woman seen in the background behind Carla) who starts to cross right then turns and crosses left then turns right and stops and looks confused as to where she is to cross. Obviously she was given directions of which way to cross and when, but forgot. (00:04:15)
Continuity mistake: After dinner Sam pours Sheila a nightcap, and he opens a (seltzer?) bottle for himself. Sheila leaves, and both her semi-filled glass and Sam's bottle are on the counter. Just as Sam opens a second bottle for himself Rebecca enters, and Sam soon sets his second bottle down near his first. In following shots the amount of liquid in Sheila's glass increases, and Sam grabs his first bottle then walks away, but when Rebecca stands up, Sam's first bottle is back on the counter and it's his second bottle that's missing. (00:20:20)
Visible crew/equipment: After Sheila leaves the bar, when Sam hops up onto the counter Rebecca exits the office, and just as Sam says, "Hey, let me ask you something, do you think my life is empty and void of any meaning," the moving boom mic casts a shadow on the wall, over the office door. (00:20:35)
Continuity mistake: While Sam considers going to see the psychiatrist professionally just to hook up with her, Carla suggests that he feign impotence, and Sam puts his bottle down on the counter, but it cuts to a closeup with Sam suddenly holding the bottle in his raised right hand. (00:09:30)
Visible crew/equipment: After deciding to decorate their own place you can see some mic equipment at the top of the screen. (00:06:30)
Adventures in Housesitting - S7-E11
Continuity mistake: When Mr. Sheridan comes to talk to Rebecca there is a pencil on the bar, when he goes to leave it has disappeared. (00:04:45)
Adventures in Housesitting - S7-E11
Continuity mistake: Sam pours Frasier his coffee - he starts to hand it to him, the shot changes, and Frasier is instantly drinking it. (00:02:50)
How to Recede in Business - S7-E1
Visible crew/equipment: After Sam walks into Rebecca's office at the start a boom mic is visible. (00:00:50)
Answer: Here is the source of the "WE WIN" photo: http://www.oldnycphotos.com/nyny587ac.html Brooklyn, of all places.