I have a number of these: the words you use wrongly but confidently until the day when some smug linguist comes along and pricks your bubble. Instead of relief at knowing the truth, you're left with a sense of deflation. Instead of gratitude, there's a double loss: you have no use for a word that doesn't mean what you thought it did, and no word for what you thought you meant. Here are the ones I could think of today; doubtless there are others.
1. Nonplused. This is definitely the one I'm most upset about. I thought it meant something like unimpressed, impassive, a kind of “yeah, so what?” When I found out it means confused, bewildered or perplexed, needless to say I was nonplused. (And shouldn't it have a double s?)
2. Hopefully. This is one of Don Watson's bugbears; since he pointed it out I still use it wrongly but now with a twinge of guilt. We can say “I hope it won’t rain,” and we might even hang the washing out hopefully, but we shouldn’t say “hopefully it won’t rain,” since “hopefully,” as an adverb, describes our attitude in performing an action. We use it more as our ancestors would have used “God willing.” Maybe I should substitute that since I can't get by without hopefully.
3. Salubrious. This totally sounds like something sibilant and lugubrious and slightly risque. But it means healthful or conducive to health. Almost the opposite of what you'd expect it to mean.
4. Lugubrious. Sounds gooey, doesn't it? It's not though. It means mournful, dismal, extravagantly, almost histrionically gloomy. Turns out there's no fancy word for gooey.
5. Refulgent. This one sounds like an expletive, a more explosive word for repugnant or repellent. You can almost feel the saliva spraying as a contemptuous orator hurls this one at some hideously refulgent object of wrath. The joke would be on him, though, since it means something more like resplendent, radiant, brilliant.
Perhaps my next list should be the words that - wonderfully, deliciously - mean exactly what they sound like they mean. Words like “obstreperous” and “malodorous.” And “unkempt.”
(No, not thinking of anyone in particular.)