I live in Europe, so we don't have GFCI exactly, the device is called RCBO but the mode of operation is same. In my flat, the RCBO is the first breaker in the panel, this way it protects whole flat, not just single circuit.
I was redoing the outlets in one room. The room is on it's own circuit, so I have turned off the breaker for this room and verified with mains voltage tester that the circuit is powered off.
When working on the outlets I had cut the cable (all three leads at once) and the GFCI tripped. I have then turned the RCBO back on and the situation did not happen again. When working on the other outlets, or the same one. I have then checked the cable with a multimeter, and there was no voltage between any 2 pairs.
Is it possible that the static electricity caused the GFCI to trip?
The installation is few months old, done to the code, and passed inspection - it was done by certified electrician. All wiring is CYKY 3x2.5 (copper leads, 2.5mm2), main breaker is 16A. The test button on the RCBO works perfectly. The room has vinyl flooring and I was wearing polyester clothing, hence the idea about static electricity.