After the installation of a logic board, a Macintosh's strange behavior or
the appearance of the Sad Macintosh may stem from the improper alignment
of the reset and interrupt buttons with the programmer's switch on the
outside of the case.
Workaround:
Remove the progammer's switch, re-insert the programmer's switch, and
check the alignment with the corresponding buttons on the logic board. If
the problem persists, remove 1 millimeter of plastic from the ends of the
two projections on the inside of the programmer's switch; these two ends
contact the buttons.