If anyone runs into this issue, here's my lesson learned. Long story short, I bought this car in bad shape dirt cheap so I could flip it. Got it in greeeaaaatt running shape, and the most recent part I replaced was the starter. Remanufactured one cuz I'm cheap. About 3300 trouble-free miles later, I turn the car off after running an errand and the starter kept spinning. Had to disconnect the battery to stop it. Obviously a short to power, now to find where. Relays shorted together? Tested ok. Pulled out both starter and starter hold relays, disconnected the ignition tumbler plug, pulled starter and ignition fuse, and re-connected the battery. I tried the preceding things one at a time to try and isolate the problem circuit. Each time the neg batt cable came close to the terminal I got massive spark and it ate a piece of the terminal. Well, MUST be the starter itself! Checked continuity of starter solenoid, and yep had continuity where it shouldn't be. There is a plunger that moves to connect the solenoid leads and activate the starter motor whenever the key gets turned. Apparently it got stuck and battery power went straight through to make a closed circuit and run the starter. Would not come unstuck, so went and warranty exchanged it for another and voila--worked the way it should. Not looking for cookies or pats on the back, just wanna pay it forward to the next guy or gal with a super saiyan starter.

yes, starter solenoids are common failures!