Just found this out the other day when I went to Purgatory on the Citadel.
So I was walking around the dancefloor and all of a sudden I fell through the ground near the dancer's platforms
I fell through this space in the floor and ended up under the map where I found coloured carboard cutouts of people/aliens

on a "dancefloor" while I could still see portions of the interactable floor above me. I then proceeded to go through ssome kind of glass wall that didn't stop me from going through (Because somehow I ended up under the bar

) and I began to fall infinitely down into a sort of pit under the map until there were only ssstrange rectangular colours and then just pure darkness
I am not sure if I could've made some kind of decision earlier to prevent my trip throughh a black hole, but oh well I used quicksave TO THE RESCUE!