New installation. Power just on. I was standing by the panel and heard a click. Mike was testing an outlet with one of those little yellow dodads and, as he was removing it, the little lights went out.
Investigation determined that the minute vibration of remocal caused one of the unscrewed in extra screws had briefly nudged the edge of the metal box cauing the trip.
Conclusion: always srew the extra screws in. Furthermore, a wrap with black phase tape will help.
~Peter