Totally believable if the debugging device was doing something with a serial port. I once hacked something together to interface a PC serial port to a Raspberry Pi. The PC serial is real-ish RS-232, with negative voltages. The Pi side was just 0/3.3V positive. I had a nice 18-volt power brick laying around, and just split it's output down the middle--what was 0 volt ground was used as -9 volts, the middle voltage was now 0 volt ground, and the 18-v line was now +9 V.
At first everything seemed OK. but when I plugged a monitor into the PI I Was Made To Realize a) the nice 18-volt PS really was high quality, and although it was transformer-isolated its output ground was tied to the wall socket earth, b) monitors also tie HDMI cable ground to earth, and so c) my lash-up now had dueling grounds that were 9V apart.
At first everything seemed OK. but when I plugged a monitor into the PI I Was Made To Realize a) the nice 18-volt PS really was high quality, and although it was transformer-isolated its output ground was tied to the wall socket earth, b) monitors also tie HDMI cable ground to earth, and so c) my lash-up now had dueling grounds that were 9V apart.