Sputnik there's a video status register, it's 16bits in length. The top 8 bits is the status (vblank/hblank/etc) and the bottom 8 is the line#. The line# goes from 0-262, and it needed to get masked so that it doesn't go over 255. As it was previously, it went over 255, which caused one of the bits in the top of the register to get flipped. Out of the many times I've played battle bakraid, I never ran into the bug. As far as I know it only happens in attract mode, not during the gameplay.
best regards,
- dink