I recently got a glowforge pro (less than a week ago) and while scoring a project last night it fell off the rails!!! There was nothing seemingly in its path that should have caused it. My board was sitting flat and within the tray. I cannot get the arm to STAY straightened out. I’ve gone through all the troubleshooting posts in here, including using the square to straighten it out and nothing. It clicks into place and it’s square, but as soon as it starts warming up and calibrating it jumps right back off. I’ve cleaned and cannot find anything to be causing this! Anyone have any suggestions? I’m already up to my eyeballs in orders and need to get this fixed ASAP. 
Check the tension of the belts on each side. They should have about 1/2" or less of movement when you press on them between the front and back.
I believe this is caused by one of the stepper motors not functioning. Or the belt on the side that is not moving is too loose
If it is bouncing off of the back when it calibrates or homes, it might be a slightly loose cable under the laser arm catching on the fins of the heat sink.
Discussion of how to check for it here:
Start reading at that post and the instructions to fix it are included farther down the thread.
Note: Please TURN OFF and UNPLUG the machine before checking for this…that’s a high voltage cable.
The belts seem to be fine, the cord is out of the way.
I’m finding the resistance is on the left side. The right side will go all the way back where the left side you can push back, but there is a slight pull that bounces it just a little bit forward.
I’m so sorry to hear about the trouble with the laser arm.
I see you’ve also emailed us and we’ve just followed up there.
We’ll continue working on this through email, so this post will be closed.