If At Last You Do Succeed - S6-E3
Continuity mistake: In the scene where Stan stops by the house to have Dorothy help him tie the bow tie of his tuxedo, the undone tie is hung around his neck. After Dorothy finishes tying the bow tie and Stan proceeds to leave, she stops him to tell him to return in an hour. He excitedly turns around towards her, at which point we can see that the bow tie is now a clip-on. (00:18:55 - 00:20:55)