At one point the crowd standing behind the salesman and Steve during the demonstration changes individual people.
When at the end Mr. Verloc is approaching Mrs. Verloc; she is staring at him straight down the center. However in the following two-shot; she is now staring at him over her left shoulder.
During lunch at Simpson's, a man's hat appears behind Mrs. Verloc between shots.
When Mr. and Mrs. Verloc are talking about Stevie, the position of the toy sailboat behind Mr. Verloc changes between shots.
When Ted is leading Mrs. Verloc to the police station, his arms go from being at his side to around her between shots.
The London Underground and tram lines had their own power supplies, both separate from the public system. A single power station failure could not affect all three.
In one shot while the salesman is brushing Steve's teeth; the crowd is laughing however there is no sound.
Near the finale, as the Professor goes to break a door open to evade the approaching police, the walls of the set on either side of the door visibly shake.
When Ted sits down to ask Verloc about the men he had met with earlier, a moving shadow of the boom microphone is visible on the wall behind and above Ted's head.