"Unlock from buffer stuck!"

[KB10Troy]

Running a cut on my engraving laser, and the gantry motion stopped, but the laser didn't turn off. The message in the G-code column was "Unlock from buffer stuck!" By the time I realized the problem, I had a small (very small) fire underway. The STOP button in LaserGRBL didn't work, so I yanked the power and put out the fire. This is, I guess, is an excellent demonstration of why a laser shouldn't be left cutting unattended.

More importantly I'm wondering why it happened, what the message meant, and what to do to avoid this in the future. I did start the engraver back up and ran a small file as a test- no problem there.

Any ideas?