The SI game has a circuit on it for assisting in bit shift operations.
It gets used by several games like SI and SID. The powerup selftest
runs an extensive diagnostic on this circuit and the diagnostic has
found that it fails in some bit patterns. I think I run like 4096
patterns through it or something like that.
There are several different variations of that circuit depending
on the board types, but the common one that fails is the one
that uses a daughter card on the sound card. If that daughter
card is not making good contact it sometimes gets reported as
a BIT FAIL by the diagnostic. If you have the type of board
that has this daughter card, then I suggesting reseating it
and/or cleaning its contacts. I think there is also a version
that the entire daughtercard is implemented in a custom IC.
There is another version where the daughter card chips are layed
out directly on the soundboard. They all perform the same
function, but done differently. The diag exercises this function.
It is possible because the diagnostic runs an extensive test
on this circuit that the circuit may "appear" to work fine during
gameplay. Or you might notice an occasional pixel from an
invader that does not get shifted correctly as the invaders work
their way down the screen. I would try reseating the shifter
board or chip. But the error is not fatal and you can still
play the multigame even with the failure, but be aware it could
mean some of the gameplay on some of the games will not always
work correctly.
-Scott