Dr. d'Armond Speers's son. Speers spoke Klingon to his son from early in the son's life, partly as an experiment. However, while the son did pick it up, afer a while he rejected it (for the same reasons, presumably, that many first-generation immigrants reject their parents' native language, especially if the parents are biingual in the language of the larger community: he could not communicate with nearly as many people in it). The other problem was that Klingon didn't have a lot of words for important baby things, like diapers.

But yeah, that's the famous and documented one.