The song that plays when you do a Mean Pacifist run where you spare everything, but you do so by lowering their health first wherever possible, and by being a huge jerk at all times otherwise- so Sans stops you in the Last Corridor to have a stern chat with you about how you might not have gained any LV, but that doesn't mean you're innocent since not committing any murder is a very low bar to pass.
I've looked up "but it's sans." so many times and got none of your videos as results when this is not only exactly what I was looking for but has been going on for YEARS! RU-vid, PLEASE!