It is worth trying but my sister got the 16 gb black wifi as an employee gift and tried to do just what you're doing and was turned down. Turns out the mini was bought from Mac Mall. It was easy enough to go down to Mac Mall and exchange.
I think there are probably some types of "Apple authorized" RAM that Apple uses in its manufacturing process, and what the Genius likely meant was that the RAM he found was not one that was installed in the factory by Apple.
Yeah, there probably is. Like I said, the computer had rebooted itself several times in a row and eventually would just beep whenever I pressed the power button to reboot.
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By the way, I apologize for pretty much hijacking this thread.