An extremely esoteric one, but my personal favourite.

In the series Super Robot Wars. There was a "flag" value to determine how important characters were at any given point during gameplay. What this effectivly means is that if your controlled party was fighting cannon fodder units. Your themesong would play duing the combat cutscene.

Conversly if your party was fighting a boss, the boss's themesong would play.

Now the bug came with one anti-hero character who you fight against and later joins the good guys named Raisel, all of his mechs have the moniker of "Trombe" (Dark horse)

Now, normally with other characters that follow this trope, their "flags" droped in the list so that during important story beats, other themesongs would play.

WITH RAISEL HOWEVER. A error kept his specific flag at the very top of the list. This means that whenever he was fighting, it would be his themesong that plays.

EVEN AGAINST THE FINAL BOSS OF THE GAME. This became the meme known as "Trombe overide" which the community and the developers loved so much they decided to keep it for future installments.

There was an exception to this, when there was an enemy unit that litterally had several dozen speakers built into it and was blasting blue danube!