If you push the debug switch after the Mac has partly or completely booted, it will bring you to the debugger (either MacsBug, or the built-in debugger). I think it's only at the disk question-mark screen where the debug switch will cause a Sad Mac 0F 00 00 0D. So maybe the debug switch was accidentally pressed midway through the boot sequence, or maybe some kind of hardware problem is causing it to act like the debug switch is pressed?

Rereading from the top: using a 12V supply instead of 7.5V might be bad news, and may have damaged something.