Yes, that's right, it only happens (1) when I don't have convenient debug data to fix it, (2) rarely, (3) when I'm running more than one process at the same time, and (4) very rarely.
Not only that but it happens in code that I don't even come close to understanding - it was written two years ago, it's dense mathematical stuff, and it ain't mine.
The boss is filling it with asserts right now (it's his) . . . I hate bugs like this.