Factual error: In Season 9/episode 21, Alan is staying at Lindsey's house when he goes to the basement to solve an electrical problem. He is electrocuted by a fuse box but fuse boxes were replaced by circuit breakers beginning in the 1950s. Lindsey's house as seen in a previous exterior shot is of modern vintage.
Continuity mistake: When anyone goes to Alan's room in Charlie's house, they always exit/enter from the left of the room entrance (when they exit the go left and up the hall then make another left). This doesn't match the floor plan of the beach house though, as the living room has been shown to be more straight out of the room.