5% of project cut and then GRBL errors

I tried to cut something pretty simple in 6061 aluminum, it seems to be ok 4 layers in and then suddenly I got an error pop-up, something about GRBL expected command letter. Carbide Motion allowed me to “resume”.

This happened two more times, few seconds apart, and then it wouldn’t let me resume anymore.

The spindle kept spinning but I cannot do anything else except to shut down the machine.

Why did this happen?

I know the circuitry inside has that USB converting AVR connected to the ATmega through normal UART. Experience from my day to day work tells me to suspect those signals getting interference.

I don’t have any more pieces to cut so I cannot fully replicate this problem again for a while