The Wedding Party (a.k.a. Sex) - S1-E3
Revealing mistake: Basil goes back to the second floor chasing after Polly. As he passes the last part of the banister, he touches the edge of it and it wobbles like rubber. (00:24:00)
The Kipper and the Corpse (a.k.a. Death) - S2-E4
Revealing mistake: When the sausages burn the first time, the smoke comes from off screen.
Chosen answer: Indeed it is Polly. She has stuffings in her mouth to pretend she is swollen.
Mortug