I've noticed some people with excellent vocabularies regularly mispronounce certain words.  I attribute this to them having read the words, but not hearing them used much.  

I'll always remember my friend Jeanne Dillard correcting our high school algebra teacher's pronunciation of some word (which I've forgotten.) Mr. Bates always said the word wrong. Jeanne has a great ear and a love for words (and a master's in linguistics and she is a successful novelist).  So she gently said, "Mr. Bates, I believe the word is pronounced "xxxx."  He turned red and sputtered, "Well either pro-nown-ciation is correct."  

The correct pronunciation makes you look smarter – at minimum people don't have to stop listening to you to mentally correct the sound.  (And they do–especially those linguists among us.)

So, here is a great post on mispronounced words.  Some of them surprised me – incluing niche – I've been "nitching" that one.