I played though Undertale, and listened to the soundtrack. There is one song on the soundtrack that I didn't recognise from the game. So where does "Song That Might Play When You Fight Sans" appear in the game?
Song That Might Play When You Fight Sans is a red herring
The song in question is exclusive to the soundtrack. The song appears nowhere within the game itself, and is intended to throw you off from noticing the real song that plays when you fight Sans:
Megalovania, which has some special significance due to its use in other forms of media Toby Fox has been involved with.
As a side note, some mods or fangames have been created that show what a comically simple Pacifist run Sans fight would be like, some of which utilize the song as the battle music for said fight.
This song can only be found in the game files, it is not used in any of the gameplay even if you try to hack it. Some people suggest it might have been a previous and now disused version of MEGALOVANIA (the soundtrack that plays when you actually fight Sans). The soundtracks are very similar when you break down the audio - there are many patterns as shown with the relation to Waterfall, Spear of Justice and Ruins. This is very possible because as good a game as Undertale is, there are still minor flaws and random secrets such as this audio piece that was discovered to serve literally no purpose in the game.