Visible crew/equipment: Wires are visible on the butterfly that Del and Rodney try to catch.
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.
Visible crew/equipment: When Del and Rodney run to the balcony, a boom mic is reflected in the window following them. (00:15:15)
Visible crew/equipment: At the start when Arthur says to quieten down, there is a reflection of movement in the butter dish between Del and Rodney, that can only be the crew. (00:01:25)
Continuity mistake: In series 4 episode 2, we see Grandad's grave. The scenery is sparse, no trees or bushes. In this episode Del is at their mum's and points to Grandad's grave. There are now massive trees and bushes.
Continuity mistake: When Del asks Rodney why Denzil lost his job, there is no sign of Denzil or anyone else behind Del. When Del turns round, the market is packed and Denzil has appeared from nowhere. (00:09:25)
Visible crew/equipment: When Rodney says the scientific name for the butterfly, a boom shadow moves over Del's arm. (00:04:40)
Chosen answer: An apparent long string of illnesses.