Other mistake: When the bratty kid walks out of the garage to chastise the girl about her bike watch the lighting around her. As she walks out of the garage it's as if someone is dimming the light around her. It abruptly gets darker the further forward she walks.
Another Song for Christmas - S1-E13
Revealing mistake: The smoke from the supposedly broken vehicle is obviously not coming from the tailpipe. It is coming from the trunk.
Mark Gordon: I got the stuff! I got the stuff.
I Was a Middle Aged Werewolf - S4-E5
Trivia: When Mark is in the motel room watching TV, the movie he is watching is "I Was A Teenage Werewolf", starring a young Michael Landon, star of this show.