Carriage overshooting target position and soft limit

Hi,
I suspect this probably is not a linuxcnc issue per se, but there is a lot of general CNC experience here so maybe someone can shed a little light.

Denford 2600, so it's lead screws all round, and steppers. There is only one physical switch per axis, which is at the maximum position for each axis.

I'm really wishing we'd added physical limit switches to the other ends of each axis.

I was running a routine to flatten the spoilboard. The gcode doesn't include any moves further south than logical 0 on the y-axis, that was set 10-15mm more from the soft limit, which itself left 10-15mm of clearance between the cutter (a 2 inch surfacing bit) and the x-axis lead screw.

So very near the end of the program there is a long traverse from the top right of the pattern to the bottom right. That should have finished with a good 20-30mm gap between the cutter and the lead screw.

I think you can probably guess what happened. One of the two x-axis lead screws now has a significant nick it it. Luckily it doesn't seem to have affected the performance, but I think it will still need replacing.

How does that happen? Even if I hadn't run the same cut twice before with no issues, I can't really see that there could be enough momentum to keep going that far with the stepper on the lead screw stopped (you can't turn the things by hand when the steppers have power).