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?