This wasn't pure Perl, was it?

No. It is a convoluted bit of XS code called from a convoluted Perl code, and only happens occasionally. and none-deterministically. Threading is involved.

I'd really like to see how your code triggered the error.

Me too. I could post everything but even if you got it to compile--it has windows specific calls--the chances of you being able to reproduce the circumstances are slim to none.

If I knew what the error message meant, even if it doesn't allow me to fix it directly, it might allow me to produce a repeatable test scenario to demonstrate it.

Besides which, asking what an error message means doesn't seem like such a bad or obscure thing to ask here?