I've had the 5v regulator burn high (12v or more) rather than burn open
dropping the output to 0v, taking every single drive and board with it.
Basically I had a metal case to use. A vendor I told this to didn't
sound surprised as he'd had this happen two or three times in the year
previous!
Well that would be a problem all right, but we were talking about
momentary mains voltage droop do to an air conditioner turning on.
Mains voltage droop is unlikely to cause the 5 volt regulator to short
out (the problem you had - outputting 12 volts).
Well designed supplies take into account the price of the motherboards
they power. There should be an OVP (over-voltage protection) circuit
on the logic supply.
Many/most computer supplies only regulate the logic to a precise value
and depend on the others to stay in regulation solely by virtue of the
transformer turns ratios - the 12 V supply can't go over voltage
because there is no excess voltage there. The logic supply has active
components to regulate the output.
The OVP circuit triggers a thyristor (SCR) into conduction clamping
the 5 volts to zero (actually below a volt), if the regulator is
shorted the OVP may cause the fuse to blow, protecting the
motherboard.
If the OVP is triggered by a transient that puts a high voltage spike
on the logic supply, the inherent current limiting protection protects
the regulator and shuts down the drive to the oscillator. That's what
was happening when I'd turn the speaker wall wart "off" on my system -
the 5 V supply would crowbar because of the transient on the 120 mains
and take all the supplies down with it - resetting the computer but
protecting the motherboard. To reset it I had to turn off the power
mains for a few seconds then power back up. Reset switch has no
effect in that case.
To cure it I put a 100 ohm resistor in series with a .05 microfarad
cap across the switch. That absorbs the power that would normally
create an arc that causes the HV transient. A ferrite bead on the
supply line was redundant but should keep any fast rise time
transients off the line.