So this weekend I had decided it was time to clean my IACV - my idle was doing a lot of wandering, and I had tried just about everything else. I removed the IACV from the throttle body, cleaned it out thoroughly (it was definitely sticking!), and re-installed it and a new gasket. I got everything back together (getting those screws tightened back up was a bear!), and gave it a start. The car started and ran just fine, although the idle was still a bit funky and I could hear a vacuum leak. So I let the car run and get up to heat while I was looking for the vacuum leak. The leak was more pronounced on a mild rev, so I gave it a few revs. I was looking for the leak for about 10 minutes or so, long enough for the fans to kick on... I gave it one more rev (no more than 3000 rpms), and BANG CLACK, the engine shuts off! Totally locked up!
Turns out, I failed to align the IACV gasket properly. This allowed coolant into the throttle body - every time I revved the engine, more coolant was dumped into my intake manifold, which then got into the pistons, which then caused something to break. This theory was confirmed when we pulled the intake.
For you own good, learn from my mistake!!!