Is there any word you compulsively mispronounce, because you got it into your head it was pronounced one way, and even though you've found out it's totally not pronounced that way, the mispronunciation refuses to unstick?
Capillary. Which is correctly pronounced KAP-uh-ler-ee, but which I persistently mispronounce ka-PILL-er-ee.
Iain is famous for these. He has one of the most prodigious vocabularies of any person to whom I've ever spoken—it's genuinely impressive. It was also gleaned almost entirely from a voracious reading habit, so he's often never heard these words actually spoken by anyone but himself, and it turns out he's not the greatest pronunciation-deducer of all time. My favorite ever was Las Vicious, not an evil city in Nevada but his pronunciation of lascivious, followed closely by you-BICK-tchoo-us, which is how ubiquitous tumbled out of his mouth.
I should note, in case it isn't obvious, that I find this habit to be one of the most charming, utterly endearing things evah about him.