A joke refers back to a conversation a scene or two earlier.
''"I swear when it's appropriate."teaser will have two or more characters chatting about something innocuous that you don't really expect to have any relevance to the plot. Sometimes the conversation turns out to be very important. Other times, though, the conversation is just briefly called back to for a quick laugh. That's this trope. Compare Brick Joke, where some detail from earlier that you may have forgotten about returns for humor value. Also compare Meaningful Echo, where an idle line of dialogue is given greater significance later.
(two scenes later)
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Indexes: Comedy Tropes, Dialogue Tropes