Yuppy Love - S6-E1
Revealing mistake: Just after Cassandra gives Rodney his coat back, she leaves the building. As she does, you can just see Rodney in the reflection in the door. He's waiting for his cue to come out of the building.
Danger UXD - S6-E2
Revealing mistake: When the dolls explode in the flat, you can tell they are being held by a member of the crew by the way they move. (00:26:30)
Chain Gang - S6-E3
Revealing mistake: When Arnie's car flashes its headlights to Del, it is obvious that the person sat in the car is not the actor playing Arnie in the rest of the episode. (00:13:25)
The Unlucky Winner Is... - S6-E4
Revealing mistake: When Rodney and Del are sitting on the balcony and Cassandra has gone to get Rodney a drink, if you look in the glass door, you can see Cassandra's reflection with the drink waiting for her cue to come in.
The Unlucky Winner Is... - S6-E4
Revealing mistake: In the airport scene, the same extra walks past 3 times wearing different clothes.
The Unlucky Winner Is... - S6-E4
Revealing mistake: After Rodney berates Del whilst he's in the bathroom of the hotel room he and Cassandra go out to the balcony and Cassandra goes to get him a drink, Del then goes out to join him. Whilst they're talking you can see Cassandra in the reflection of the balcony door waiting for her cue to come in to give Rodney his drink.
Revealing mistake: When Rodney is asking Del if he thinks the bathroom is haunted, you can see stage lights in his spoon. (00:10:20)
Revealing mistake: As Del is sitting down to watch his video, if you watch the video recorder, a false top superimposes over the real one.
Chosen answer: An apparent long string of illnesses.