Sorry for not having provided more information earlier. The bug seemed to only appear
on my work machine (and I've been to busy at work the last few weeks to provide more
information), but now I can reliably reproduce it on my home machine, too, funnily
enough after a clean re-install of Arch Linux.

It certainly seems to be the same bug as the Gentoo bug #560158 you linked. I cannot
reproduce the behavior in either gdb or valgrind, but without those the command fails
every time.

I've since downloaded and manually built 0.9.5 and 0.9.6. I could reproduce it in the
standalone 0.9.5 build (again, neither valgrind nor gdb could be coerced into also
showing the behavior. The issue seems to be fixed in 0.9.6, though! Arch doesn't yet
provide the new version as an official package. I'll tell you if the issue's also fixed
when that package comes out.

Still have no idea what caused it though.