aarg:
I recently applied 12V to the 5V of a mega because of an accursed non-standard adapter that produces 12V on a USB connector. I pulled the power when things didn't work but it was on for a few seconds. But it still works now, at least for the time being...
Whatever puts out 12V on a USB connector should be crushed now! Few seconds doesn't matter,
over-voltage damage to CMOS happens in a fraction of a microsecond.