Revealing mistake: In the scene in the kitchen, Lucille Bluth takes a bottle of "Bushmills" (a brand or Irish whiskey) out of a grocery bag during a scene in the episode; however the liquid inside is clear/transparent and obviously not whiskey.
The One Where They Build a House - S2-E2
Revealing mistake: When Michael is talking to George Michael, Oscar, Buster, and Tom Jane about the house they are about to build (and what they all want out of it), it is clear in the wide shots showing all five that Tom Jane is not present; the person standing in for him has a completely different haircut and wears glasses. They try to hide it but it is obvious at several points. (00:14:15)
Revealing mistake: When Lucille 2 is getting a drink, the bartender goes to pour liquor from a bottle into a cup of what looks like club soda, but nothing comes out.
Chosen answer: "The One Where They Build a House," S2E2.