Every geek has had the experience of saying a word aloud that they learned from reading &mdash and discovering that they pronounce it wrong. It's rather a rite of passage for young adult geeks1. It's been a few years since I experienced this phenomenon from the speaker's side. But yesterday I found myself using the word "fecund" in a sentence — and just as the F sound started passing my lips, realized I did not know how to pronounce it. Which isn't to say I didn't have a pronunciation all loaded up and ready to emerge from my lips. But I had no idea whether it was, in fact, the normal pronunciation2. This tangential thought caused me to screw up the bit of mental arithmetic necessary for the rest of the sentence, such that I said "August" when I meant "May".
(Perhaps the threat lies not in a large vocabulary, but in the meta-state of observing yourself stumbling over your own vocabulary and thereby menacing your arithmetic skills.)
1 I do wonder, with the advent of computer dictionaries that will pronounce a word for you at the click of a mouse, whether we may be the last generation of nerds to share this common experience. It is both vastly quicker to look up a word online than in a paper dictionary, and much less work to click your mouse than to figure out your dictionary's pronunciation key. I'm sure if I were a kid today I would use an online dictionary far more than I in fact used paper ones when I was little. (I was always very fond of dictionaries, but it was nevertheless rare that I wouldn't try to puzzle out a new word from context first, and only turn to a dictionary if I couldn't. Or — rarely — when eiher zero or two obvious pronunciations occurred to me.) This is, of course, not an unmixed blessing: We may also be the last generation discover the pleasure of browsing the dictionary.
2 According to thefreedictionary.com I in fact mangled it rather badly.