Oh, the high & low screw stops were fine -- I couldn't shift off the gears no matter what. I could tug on the derailleur cables and the chain wouldn't fly off. In fact, the rear's High screw had been saving me on the road; since there was so much slack in the cable, if the screw hadn't been spot-on, the chain would've fallen off the small cog.

It was all the shifts in between that I had screwed up. Trust me when I say that I really messed with the barrel adjusters. I didn't know that they only needed small turns, so I practically unscrewed them completely -- I might've had them twenty or so full turns in the wrong direction.

While plucking the cables, I could tell when I was adding just enough tension to "match" the levers to the derailleurs. For example, with a slackened cable, the right shifter had to move a bit just to take up the slack and move the RD, which threw off the indexing for further shifts. But, with the slack already taken out, the shift lever's movement went right to moving the RD, and the index clicks finally matched up with the rear cogs.

Frankly, I'm just relieved that I didn't break a chain before figuring this out...