Revealing mistake: During Jack's dance scene, the old lady that he dances with spins away from him and knocks off a man's toupee. The actress was supposed to knock it off as she went by, but it's obvious that she misses and instead quickly reaches over to snatch it off his head instead.
Episode 70 - S4-E1
Revealing mistake: Toward the end of the episode, Bert receives an envelope with pictures of him and Danny at a motel with some women. As they look at photos, Bert hands Danny some photos. Danny crosses left. Bert turns stack of photos over and says there is a note. He reads note and turns photos back over afterwards. The backs of the photos are blank. Danny crosses right to Bert holding photos. You can see on the back of Danny's photos the note Bert just read from the other side of the room.