In the very beginning, on one of my UM2 printers, I had the Z-axis moving down a couple of times during long prints, due to overheating and temporarilty shutting down of the drivers. In mid-print it would fall down 5mm, and then continue printing as if nothing had happened. This was only on long prints, or very intensive use all day. I never had it on the X- or Y-axis, but I can imagine that it is possible too.
In my case the solution was to reduce current through the stepper motors. Since then, it never happened again in all those years. If I remember well this was done on the pri