I was getting some weird intermittent behavior with Pearl today while cutting acrylic. Two times during two separate cuts the laser head jolted back, swerved on a straight cut, then continued in the offset position and never allowed the head to travel as far up in the Y position afterwards. It seemed to jump by 1/4" each time this happened because the ending offset pictured below was about 1/2". The lens looked fine as far as I can tell. @dannym any ideas here? If anyone uses it in the meantime, proceed with caution. I ruined a significant amount of expensive material with it 
That last pic shows the max position the laser head would travel for setting the origin to the top left corner of the bed. Each time the mess up happened the offset increased and it would not allow me to adjust it further up.
Most likely prob is the focus stick popped out of it hole and the gantry can hit it.
I took a look and that was the first thing I noticed
That’s very strange, I definitely put the Z stick back in like normal. Why would the laser head no longer travel the same distance in the Y direction afterwards? It made no unusual sounds, just refused to travel further. The issue also occurred in two separate areas of the bed (first time was roughly 12",7" and second time was close to 0",12" — speaking in X,Y). If it’s helpful, I can get more precise info on those coordinates. I might also try doing some tests on it today with scraps and see if the issue occurs again when the Z stick it not stored inside the machine.
Those are open-loop steppers. If they encounter enough resistance, they can stall and the system will not know. This rarely results in “losing steps”, once a motor is stalled it generally won’t start again until the commanded speed crosses zero again, so it can’t just lose tiny amounts.
It’s possible that a worn bearing can cause enough resistance to make a stepper stall. And it’s definitely been long enough for bearings to potentially wear out.
I think the Z-stick has shook itself out of the hole enough to hit the gantry on occasion.
I did have it powered off and pulled on the gantry to see how smooth it was, and the gantry hit the stick right off the bat. I think the stick was turned so the flat doesn’t line up with the extrusion beside it so it didn’t lay in the hole as far as it should. I didn’t notice anything else at the time.