The Way We Almost Weren't - S2-E23
Revealing mistake: Closer to the end of the episode, when Lilly and Oliver are lying on the floor glued to the chairs, you can easily see black strips on their backs attaching them to the chairs.
Revealing mistake: In the opening credits, Beefy Bert has a different hair colour to what he has in the actual episodes. This was not included in Series 3.