In the original Super Mario Bros. NES game from 1985, certain short-term events, and the sounds associated with them, often seem to cause fluctuations and distortions in whatever background music is playing at the time. For example, when one picks up a coin at the same time as a note in the background music begins, the note often modulates to a different pitch or is absent altogether after the coin pickup sound ends. Jumping on or defeating an enemy also often produces distortions in the music. Why is this? Is there an explanation related to the sound or hardware limitations of the NES console?
(Admittedly this question may be misguided, as my NES broke several years ago and I've only been playing the Wii Virtual Console version of Super Mario Bros. ever since. But I do seem to recall the glitch happening on the original game, and it's not like they couldn't remove these distortions in the modern version if they wanted to.)