Revealing mistake: When Basil is climbing up the ladder to view the inside of Mr Johnson's room, he accidently peers inside Dr Abbott's room. When he starts doing his "checking" routine on the window before he falls backwards, the pane flexes, indicating that it's made of clear perspex.
Revealing mistake: When Basil sneaks into Raylene's room to listen for the "extra guest" in Johnson's room, Raylene wakes up screaming. When Sybil comes in, turns the light on and Basil says "I didn't know she was there, I just came in to check the walls", look at the extreme right of the screen. You can see the shadow of a boom light on the wall.