Have any of your languages evolved exceptions... by accident? I've been writing merrily along using the word _lei_ "person" in the third declension, for nouns that end in "a", stressed vowels, and diphthongs. In the plural, that makes it: Nom: leija Erg: leijar Abs: leijan Gen: leijas Loc: leijat Dat: leijam However, I just remembered today that "ei" isn't actually a diphthong in Lyanjen! It should really be in the second declension, whose plural runs: Nom: leau Erg: leaur Abs: leaun Gen: leaus Loc: leaut Dat: leaum However, I'd already used it so many places that I was loath to change them all. So I said, fine. Lyanjen has just gained its first exception! And it was duly noted. Any other such stories?