I really appreciate the work on improving game rules in older games, but please let this thread be for actually documenting bugs in original software, to help hobbyist developers finding out what needs to be fixed.
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Alien Poker - maybe other System 6 games also: If ball bounces while landing on a hole kicker, triggers but misses the kickout arm and then settles at hole, the game will not recognize ball in hole, requiring power cycle to continue. Back then, this happened so often that I wired an extra EOS switch to one flipper, breaking the circuit of top hole switch. Then if the ball got stuck in top hole, players instinctively flipped, getting the game to notice the ball in hole and kicking it out. All 3 eject holes behaved like that, it was probably because of playfield layout that the top hole was the only one that really suffered from the problem.

Good thing that the byte in $12FC was not $DD, the infamous "halt and catch fire".

Sometimes in a Bally -35 game some adjustment that has range 00-03 (e.g. the sound adjustments) does not operate correctly even if the displayed value is OK. To fix, I keep the start button pressed to let the value go from 03 back to 00 twice and then it seems to work.

Usually this happens with an uninitialized CMOS RAM after battery or 5101 change.

Maybe, even if the setting uses only 2 lower bits, it only clears the upper bits when going from 03 to 00, and when testing the value it includes the random upper bits.