In the code for the "Alien Auto-topsy: Part 1" mission, there is a command called "AllowRockOut()" which can be triggered after collecting the nuclear waste barrel and parking the car while Homer is a passenger and someone else is driving (this can be performed while Homer is the driver, but is reportedly trickier to pull off). Homer can then be seen "rocking out" to the mission's music by performing the "Task Completed" animation over and over again (any other character animations will temporarily override it). However, if you cancel the mission during this animation loop, it will be revealed that the speed he rocks out at changes to accompany the tempo of the song that is currently playing. The RMS files (which control music events) contain tempo data for every song in the game, despite mission warping effectively causing rocking out to go unused.
Contributed by MehDeletingLater