This happens when you have an unanchored part that you’re setting the CFrame for. Any parts on top of it will just slide around even if you keep the velocity and rotvelocity set to 0,0,0 and you keep the part in the same CFrame all the time. I think it’s always happened.
Repro place (press run): Dropbox - Error - Simplify your life
I assume this is happening because the parts fall down expecting the unanchored floor to fall too, and when the floor doesn’t fall the parts get bumped back up and appear to be sliding. Notice how the parts sink into the the floor as well.