When a person not involved in telling joke interrupts the person who is telling joke, thereby flummoxing the person and causing the mistiming of the punch line.
Kevin was just about to utter βTa-da,β the punch line, when Rob interjected with a question, only to punch linecock block Kevin.
A line you say to someone to put them off gard before you sucker punch them.
Among the lines are,ever hear of an old man by the name of?, I droped my card could you get it for me?, Your fly's open, your shoe's untied.