When I have had this happen it seems to be when there are some lag spikes happening. I assumed it was from moving on a nonflat surface and kind of lagging into the ground while the server tries to resync but that could just be me making up stuff in my head. When I got stuck last time there was a constant stream of "invalid move attempt" messages even when i wasn't moving and I had the same snap back that others were reporting until I relogged.