Revealing mistake: When Del sees Denzil skating along and tells him that he's got his money (the butterfly), Denzil gives him a high five and crushes the butterfly. If you look at Del's hand when Denzil goes past him, you can see the butterfly is not crushed at all.
A Slow Bus to Chingford - S1-E5
Visible crew/equipment: When Del says to Rodney atop the bus about Janice not wearing a bra, a stage light is reflected in Del's drink. (00:25:10)
Revealing mistake: When towing the car, the tow rope is slack, showing the van is not towing the other car. (00:05:45)
Watching the Girls Go By - S4-E6
Revealing mistake: At the start, Mickey Pearce is playing darts in the pub by Boycie. On the 3rd dart, he actually aims for the floor behind Boycie to make it seem like the dart bounced off the board. (00:01:10)
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)
Yuppy Love - S6-E1
Continuity mistake: The text on Rodney's computer keeps changing at the start with no one touching the computer. (00:03:15 - 00:10:50)
Visible crew/equipment: Just before Del gives Rodney his cup of half full tea, when they are in the middle of the sea, a boom mic is visible at the top of the screen.
The Unlucky Winner Is... - S6-E4
Visible crew/equipment: When Rodney is talking to Cassandra in the pub, the shadow of a crewmember/camera is visible on numerous occasions behind Rodney.
If They Could See Us Now...! - S9-E1
Visible crew/equipment: Just as Rodney says "Unbelievable", when he realises he was to blame, as the shot changes to Del, the boom mic is visible in the top right corner for a split second. (00:13:30)
Continuity mistake: When Rodney drove the van to help Del escape from the police. When you see a shot of the van from behind, the van has changed position. (00:03:40)
Continuity mistake: When Del is pretending to be ill, Albert and Raquel say that they called the doctor in the previous evening. However, a few shots earlier we learn that Del was out with Rodney the night before.
Revealing mistake: In the opening scene, there is no light switch in the Trotter's flat.
Continuity mistake: When Rodney comes into the dentist to see Del about the gerbil, he looks a bit windswept with his hair everywhere, but when it cuts back to Rodney, his hair is perfect.
Continuity mistake: When Del puts the painkillers in his drink, it changes to a lighter colour. Yet when he picks it up off the TV a few minutes later, it is back to it's normal colour.
Visible crew/equipment: When Cassandra is on the phone to Del at the start, a boom mic shadow is visible.
From Prussia with Love - S5-E1
Visible crew/equipment: When the German girl asks if she can sit down, whilst in the Trotter's flat, just above her head, a boom mic pops into view.
The Long Legs of the Law - S2-E1
Visible crew/equipment: When Rodney walks over to Grandad and Del, just after Grandad says "they are never off duty", a boom mic pops into view for a second and then moves back up again.
Visible crew/equipment: After Albert has filled up the baby bottle with the proper water, Myles, Rodney, Del and Albert are all back in front of the shed. In one of the shed windows, a boom mic can be seen moving around.
Continuity mistake: When Del is getting chased by the dogs in the alley, some of the dogs run a different way, yet in the next shot, they are all chasing him again.
Chosen answer: An apparent long string of illnesses.