Quoted from tjsynkral:
You missed a few more machines which were turned on and looked nice but had 0 credits and no keys or credit button. I turned those off when I found them but they kept popping back on but not getting fixed.
That's sad, since there's numerous workarounds, including a physical credit button. One arcade I went to had credit buttons that were paper thin, and worked every time, no drilling needed.
...or set the replay score set low. I felt like a winner at MGC with the replay knocker hitting.
...or have someone with a key walk by every x minutes, and flick some credits on.