I used the phrase 'an hypothesis" below, following what I thought was the prescriptivism I was surely taught in 6th grade that a noun starting with the letter "h" should be preceded by the "an" form of the indefinite article.
But it didn't seem right, so I Googled the two versions. The results are thus:
Results 1 - 10 of about 756,000 for "an hypothesis".
Results 1 - 10 of about 2,070,000 for "a hypothesis".
And so wins "a hypothesis" ... for now.