This seems to have happened several times at Regionals, sometimes due to
human error but I didn't see any hands near the ball chutes in this particular match.
Some Regionals posted human scorers at each goal.
I haven't looked at the position of the ball detector beams, but if there is a situation where multiple balls only break the beam once, then there could be conditions that would cause a negative count.
I wonder what the FMS does when more balls are returned to play than it has counted as scored?