It turned out to be my fault. I had made an error about what case was most common in code that I had written before.
The kicker: I had made this error in early March, 1998. (It was in RLEWeights, for those of you to whom this means anything.)
I'm not sure anyone had looked hard at the implications of this circumstance before.
There's a certain argument for moving on to other codebases before things like this come to light.
(What's the oldest bug of your own that you've found?)