I have an Avid CNC machine that I'm having issues with and I can't figure it out on my own.

We cut 10 different patterns on the machine, over and over. Every once in a while, the machine will cut about a half inch deeper than it is supposed to, destroying the table. I'm not the one operating the machine so it's even harder for me to troubleshoot. From the little I know about stepper motors, I don't know what would cause it to gain steps. Every time I operate the machine, it works correctly.

Any ideas?