Only place where "BUST" and 20 points are related is at "2nd player 10 point card" - 2nd increment.

There is a 2 way relay at "A player relay"... as far as i can tell, this is the only path to ground for 20 points.
If A player is misaligned so it is NOT opening one side of the contacts, game will bust at 20.

OR

2nd player 10 point card is not resetting to ZERO when its meant to