i usually model with multiresulusion modifier . sometime i want to go back to previous level to make some corrections but then his is what i get- some bumps around the surface
Unfortunately Multiresolution is buggy in Blender, what happened to you is one of the most annoying bug, because it can’t go away.
a month ago Psy-Fi asked people to add the bugs they found with multiresolution to gather case ( with a blend ), in the hope one day these will get fixed :
https://developer.blender.org/T40605
Maybe you could add yours