Revealing mistake: During the end credits of this episode, a bowl of soup is shown. A frog appears in it for a few seconds and then sinks under the water again. You can see the string being used to pull the frog up. It surfaces as the frog does, and is very easy to spot.
Revealing mistake: When the staff are assembled in Mr. Rumbold's office sans Mr. Lucas, Lucas comes rushing in late. However, before he does you can see him standing just outside the door, waiting for his cue to rush in.