nauseated vs. nauseous. Okay, granted, I didn't used them right until I started writing, but now it's so deeply ingrained that it irritates me when anyone uses it wrong. It took me a while to remember how to use this one, but I remember it to being similar to "poisoned" vs. "poisonous". The former means that you are being affected by poison (or nausea), the latter means that you are affecting others with poison (or nausea). So if you say you're nauseous, you're actually making other people feel ill.
I did enough presence of mind that, when Heather was experiencing nearly constant nausea during the first half of her pregnancy, and would tell me she was nauseous, that I did not even once correct her. Because one of the the last things one wants to hear from one's husband when nauseated is a grammar correction.