I have a family of fonts I'm building in FontLab. They are all using the same exact kerning data and kerning classes. For some reason, one of the fonts is giving me a "serious error" in Font Book that says there is a "System Validation" error, with no further details.
I've tried taking a .vfb file from one of the working fonts, copying over glyphs from the font that's giving me trouble, and generating again. Same error. I think I've pinpointed the problem to be something kerning/class related, as if I copy/paste glyphs that AREN'T in a class and generate the font, I don't get an error in Font Book. But once I copy glyphs that are in a kerning class, I start getting the error.