I cannot believe that so many people have read my question, yet no one has any clues. Have I discovered a truly SINGULAR error?
More on the mystery cymbal hit: if I select all and delete every note in the affected regions, the cymbal remains. If I delete the region, the cymbal goes away. The hit is also in the same place in each region, because when I created the drum track, I copied and pasted the first region, then changed the notes around.
The funny thing is, I've heard the song so many times with the extra cymbal hit, and it occurs in not-too-bad a place, in the drum fill at the end of the measure, that I've kind of grown accustomed to it, and think it actually sounds kind of cool where it is. So I think I'm just going to incorporate it into the song. Maybe title it "Random Cymbal Crash."
Anyway, thanks for reading. I would still be interested if anyone knows why this has happened.
Kishin