I have seen mixed outlines before, somewhere. ufo2ft would convert cubics to quadratics for TTF compilation and leave the existing quadratics alone. Just a random thought :)
Interesting, thanks. I think you may have seen them because of a FontForge bug. MFEKglif is very anti-mixed contour, for good reason, but there's no reason glifparser needs to panic instead of converting.
https://github.com/MFEK/glifparser.rlib/blob/e443ccbd4d33c73654c342b268e36e44b62f6b71/src/outline.rs#L25
I have seen mixed outlines before, somewhere. ufo2ft would convert cubics to quadratics for TTF compilation and leave the existing quadratics alone. Just a random thought :)