What Happened to the Mouse?/Music

Möte i Monsunen, a song from 1935 by the Swedish musician Evert Taube (known to write all manner of gibberish and getting away with it because of his status as a national hero) has an ending very reminiscent of this trope. A sailor by the name of Fritiof encounters another sailor, and tells him of when he sailed across the seas with wild animals on board. A few of the ending lines would be something like this, translated from Swedish: "But, Fritiof, the elephant, what became of it?" "When we meet again, I will give an answer to your question" and then promptly the conversation gives place to a description of how Fritiof rows back to his boat.
The Big Pun song Twinz has an opening verse about a gangland hit gone wrong that is so catchy that it pretty much makes the song. Once the listener is told that they killed the wrong man, the hit is never mentioned again. Even worse, the video for the song covers an alternate plot that does not exist in the song.
Justin Bieber, in his song Omaha Mall, goes to check out some girls. He says they look good, but you never know in Omaha Mall and suddenly girls are never mentioned again in the song.
The Lamb of the Genesis Concept Album The Lamb Lies Down on Broadway is mentioned only once, in the very first song on the album.
In James Blunt's "You're Beautiful", the first verse ends with the line "...but I've got a plan." We never find out what this plan involves, or if anything happens as a result of it.