I let an idiot at work play with my mini, and this is what he did:
plugged it into his ancient 15" dell lcd monitor, brought up the display System Preference, and clicked on the "Mirror Internal Display".
the internal display on the mini 9 wigged out and started displaying dozens of vertical lines in a multitude of different colors.
now, whenever I connect ANY external display, the internal display does the same exact thing, except, it also mirrors that same wigged out display to the external display.
I have deleted the /library/preferences/com.apple.windowserver.plist file, that didn't help, and I have also tried copying the same file from a mac mini with no change. the blue function keys do work as expected, they will switch the wigged out display from the internal, to the external, to both, but alas, it is always the wigged out display.
does anybody know how to fix my machine so that mirroring is turned off?