As Keel and Steed rush to de Willoughby's aid during his beating, a bespectacled member of the production crew can be glimpsed in the background before ducking out of shot.
At 46 minutes in, the shadow of a boom microphone is visible on the back wall on the left side of the screen, then again above Mrs. Briggs as she sits down. The microphone itself is seen in top left corner when Jeremy shouts, "I'm not your son!"