@dc42 In case it is still useful to you, you can also cause Duet to run out of buffers by slicing a large object with PrusaSlicer , having enabled the Printer Settings->General->Firmware->Supports Remaining Times option.
This option puts M73 commands into the gcode which only Prusa printers support. Duet will throw errors and the output buffers will eventually be used up. This can take a while, of course.
This was happening to me when I mistakenly selected that option, and the web interface would disconnect part way through a print, and I could not reconnect until I cycled the power on my Artemis printer. I had seen the "M73 command not supported" errors, but I ignored them until I figured out they were causing the buffers to be consumed.