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.
Read more about how to solve these issues in Glyphs App here.