Handcuffed - S4-E18
Plot hole: Chrissy's policeman cousin informs the roommates that there was a complaint from a neighbor about her husband on his roof watching the girls dance in The Apartment with binoculars. However, when the girls were dancing, the front door was fully closed and the very thick drapes were fully closed so how could such a complaint exist?
Handcuffed - S4-E18
Continuity mistake: When Jack is eating cake and Chrissy is under the table her the sleeve of her right arm is down. When Jack and Chrissy exit the kitchen into the living room, both of her sleeves are down.
Answer: I believe you are referring to the artwork of Alphonse Mucha. On the wall next to the front door was "Feather" (which is one half of "Primrose and Feather"). Above the telephone in early episodes was "Zodiac". Also seen through The Apartment was "Spring" and "Autumn".