The One After Ross Says Rachel - S5-E1
Revealing mistake: When Phoebe phones Emily's step-mother to say she is Ross' doctor, she looks out the window behind Phoebe. The buildings are moving slowly from side to side. (00:04:10)
The One After Ross Says Rachel - S5-E1
Revealing mistake: When Chandler and Monica (and later Joey) are talking on the plane on the way back from London, the little display screen (showing a map of the Atlantic and coastlines) in front of Monica's seat shows the little grey plane indicating their position and direction. The plane is facing east, as though they are travelling towards London, but they are supposed to be flying back to New York. (00:14:45)
The One With The Triplets - S5-E3
Revealing mistake: When the doctor says, "Here comes the head," it is very obvious that there are feet coming out. (00:14:50)
Revealing mistake: When Monica and Rachel apologise to Danny, as he shuts his door for the second time the set wall visibly shakes. (00:07:55)
The One Where Ross Moves In - S5-E7
Revealing mistake: When Joey and Chandler go in the bedroom to talk, Joey suggests they play a game of foosball. There is no ball when they start playing - quite noticeable when Joey " scores" a goal.
The One Where Ross Moves In - S5-E7
Revealing mistake: During Danny's party you can see Monica's apartment through the gap between the door and the wall, despite the party taking place on a lower floor. This is because they just reused the set for the hallway to save having to build a new set for the "lower floor."
The One With All The Thanksgivings - S5-E8
Revealing mistake: When Joey has the turkey on his head, and Chandler is by the door, look at the wall to Chandler's right. It's a "wild wall" - i.e., a movable wall put there to extend the set outward an additional few feet, since the camera normally doesn't see that far forward. The wall angles oddly away from the room, and you can see how the line and pattern of the hallway floor continues into the room. (00:06:15)
The One With Ross's Sandwich - S5-E9
Revealing mistake: Joey's bucket of chicken is, rather obviously, a KFC bucket with "Finger Lickin' Chicken" stuck over the original KFC lettering - you can even see the picture of Col. Sanders. (00:17:10)
The One With Rachel's Inadvertent Kiss - S5-E17
Revealing mistake: When Ross is doing his "Watching TV" bit, there is a brief shot of the blank screen that he is sitting on the sofa pretending to watch. In the screen's reflection, it is clear that the sofa is empty. (00:03:55)
The One With Joey's Big Break - S5-E22
Revealing mistake: When Joey and Chandler are traveling in the car, the background is a very obvious projection. (00:11:45)
Answer: The painting doesn't have a name, and if it does it is never mentioned in this episode.