Continuity mistake: After Fred secures the chain lock to the front door, with the chain mounted onto the frame of the door, Fred then slides the chain into the door's slide-catch to show Lucy the "burglarproof chain." When Ethel opens the door the chain itself is ripped off the frame, leaving the chain (and the piece that was screwed to the frame) dangling from the slide-catch attached to the door, but in Ethel's closeup the chain is still mounted to the door's frame.
Picture added by Super Grover
To submit a picture, just click the edit icon under the relevant entry, then choose 'add a picture'. Thanks!
All images remain the copyright of their original owners - these low resolution images are simply individual frames used to demonstrate the entry.