I've never seen this before, but it's popped up a half-dozen times today. No rhyme or reason to when it does it -- it's happened while editing a character, while kerning, and while trying to save a file. I'm guessing there's something amiss inside the font itself, but the error isn't very clear on what's wrong.
The font I'm working on is one where I've taken one of my previous fonts, deleted some glyphs (all either mapped to the PUA or generated by the composite wizard). I've re-run the composite wizard, but that hasn't stopped the error from popping up. And I never got these errors when I created the original font.
Following up: just had the same error happen with a brand-new font. I'm wondering if this is something having to do with the Windows 10 Anniversary Edition, which I just upgraded to (not really by choice) a couple of days ago.
64 bit. I'm pretty sure it's the update now -- I rolled back to the previous build (Win 10 #1511) and it was smooth sailing all day, not an error to be seen. But Windows went ahead and scheduled another install of Anniversary Edition (build #1607) overnight, so I'm rolling back again this morning. Hope I can figure out a way to keep it rolled back, because Type isn't the only program that's problematic -- NexusFont becomes unusably slow in the update too.