Comment by duckerude

RFC 3629 says surrogate codepoints are not valid in UTF-8. So if you're decoding/validating UTF-8 it's just another kind of invalid byte sequence like a 0xFF byte or an overlong encoding. AFAIK implementations tend to follow this. (You have to make a choice but you'd have to make that choice regardless for the other kinds of error.)

If you run into this when encoding to UTF-8 then your source data isn't valid Unicode and it depends on what it really is if not proper Unicode. If you can validate at other boundaries then you won't have to deal with it there.