Bugs can be absolutely pathological.

This is very true. I simplified the information a bit because I was posting about the math as a matter of intellectual curiosity, not to get help debugging. I have a model of what was causing the crash that I find reasonably convincing, which I outlined in my response to jimrandomh, below. So, while it's a real-world problem, for purposes of math we can assume that the effect of commenting out the line is an indication of a point bug, as it were. I found the Bayes confusing anyway, so there's no need to complexify further. :)

Thanks for the math answer; I need to think about it carefully to absorb it fully, but I thought I'd respond to the programming answer first.