Heh heh heh. Going on words I had read and knew (but somehow could not SAY correctly) - gelding. For some silly reason, my young self was POSITIVE that it was pronounced "GEEEE-ll-ding" My mother could not convince me otherwise. I'm not sure what did change my stubborn head, but I think it was years....
My other word - heuchera - a plant I've seen numerous times at numerous nurseries - I was SURE it was hwe-CHER-a. I heard somebody say it, and then got up the bravery to ask nursery staff "HOW do you say this name??" YOU-ker-a. Who knew?
I feel endlessly stupid when I find I've mangled the pronunciation of a word, but I can, 99% of the time, use it properly, I just can't SAY it properly - it's all that reading that is at fault. Only the knowledge that many of my friends and co-workers also mangle words (both meaning and pronunciation) makes me feel less stupid!
Bonus - I was a really good speller until I took Spanish in 7th grade. After that, I couldn't separate out English spelling and Spanish spelling.