The Spanish word for “bat” is an official typo

In medieval Spanish, a mouse was a «mur». And a certain mur that flew at night, seemingly unable to see, was a «mur ciego» — a blind mouse. That gave «murciégalo», the etymologically impeccable form.

But speakers kept swapping the L and the G — a phenomenon linguists call metathesis — and «murciégalo» became «murciélago». The mistake spread so widely that it became the standard form. Today the dictionary lists both, but say «murciégalo», the original, and people will look at you as if you'd got it wrong.

As a bonus, the triumphant typo became a puzzle-page celebrity: «murciélago» contains all five vowels, each exactly once.

Languages aren't fixed by scholars. They're fixed by speakers, getting it wrong with admirable consistency.