Are any paths in your characters overlapping each other? Not good! These need to be joined together into a single path (one single outline). Otherwise, these overlaps will cause output errors.
Among the usual suspects are glyphs made from two parts: Œ œ Æ æ Ç ç Ð ð Ę ę Ø ø ¥, etc. A common error often occurs with diacritics. Designers may remove the overlaps in a basic glyph — for instance æ — but forget to do the same for derived glyphs like ǽ and ǣ.
Also, watch out for incorrect path directions, open paths, and accidental duplicate outlines on top of each other.