Yesterday before leaving work, I left a bunch of parts to print on our Prusa Mk4. This morning, I found all the parts with the same defect at the bottom: apparently the magnetic printing plate shifted ⅛¨ sideways on the bed 3 or 4 layers into the 7-hour print.
Unfortunately, I removed the magnetic plate without paying attention before I saw the defect, so I'm not really sure if this is what happened. It stands to reason, but it might also be both stepper motors somehow skipping at the same time. It seems unlikely, but maybe there has been a power brownout or something.
Or someone at the office interfered with the bed's movement at some point. But it seems unlikely too.
If the plate itself shifted, maybe it's because I had cleaned it with water just before and I may have left some slightly damp spots underneath, despite drying it as good as I could. I might also have failed to position the plate against the registration pins at the back of the bed, but I usually pay attention so that too is unlikely. Still, I might have been careless.
Has anybody had this sort of thing happen before? This is the first time for me.
The plate does indeed stick very strongly against the bed. The magnets are fine.
The most plausible explanation I have is that someone stuck their hand in the enclosure for some unexplainable reason and the bed hit their arm or something. The only problem with this theory is, there is zero reason for anybody to do that. But I did start the print early enough for a few people to still be at work when it happened.
Other than that, I've taken to using glue on the plate lately, because I've had adhesion problems. Maybe some leftover glue turned liquid-ish and degraded into some sort of lubricant with the heat (I'm printing PETG with the bed heated at 85C) and it seeped between the bed and the plate. I've never felt it was ever slippery though.
Or it was some leftover water that steamed over between the bed and the plate and lifted the plate for a second, air-hockey stylee.
None of this seems very likely though. So I cleaned the bed and the plate real thoroughly, installed the plate tight against the registration pins, upgraded the firmware to the latest for good measure, slathered glue super-carefully all over the plate, making sure it didn't go overboard, and started the same print once more. I'm watching it remotely through the webcam I installed in the enclosure and it seems to be going fine.
Belts look tight, but a good inspection of them and their pulleys might tell you something.