A linguist predicted a sound no one had heard — then it was found in Hittite

In 1879 Ferdinand de Saussure noticed odd vowel patterns in Indo-European languages and proposed lost consonant sounds to explain them, sounds that existed in no known language. In 1927, decades after his death, Jerzy Kurylowicz found that very sound preserved in newly deciphered Hittite tablets, confirming the prediction.

— Ferdinand de Saussure; Jerzy Kurylowicz, Laryngeal theory (confirmed by Kurylowicz, 'e indo-europeen et h hittite') — Symbolae Grammaticae in Honorem Ioannis Rozwadowski, 1927