I'll pick up Pro Tools eventually, but I really like Logic.
Anyway, the first time it happened, I had unplugged the amp from the computer (I pulled the cord out of the amp), and strummed the guitar a couple of times to test the volume of the amp. Then Logic started making this super freakin' loud SQUEAL, like a feedback squeal. Nothing would stop it. Muted the speakers on the Mac, turned off the speakers, was still doing it. I finally quit Logic and it stopped (which is how I know it wasn't the amp).
Second time, the instant I pulled the cord out of the amp, it started doing it again. Again, I quit Logic and it stopped. But it was a DEAFENING squeal that hurt my ears and gave me a headache. Hell, my ears are STILL ringing from it.
(yeah, no sh--). Still, I hope this doesn't keep happening.