I find it supremely annoying when I fall or cough and someone asks "are you okay?", whether with genuine interest or as a knee-jerk reaction. (It's much worse if I didn't even fall and it was something even more minor, like lightly bumping into something or tripping and catching myself before I fall.) I know that I have fairly little right to be annoyed by someone attempting to care for me or being otherwise interested in my well-being, but it still drives me nuts.
I suppose it's the implication that I need help that bothers me, though I'm not entirely certain. I don't want it said or implied that I need help, and I don't want to ask for help unless it's really necessary. When I slip on a stairway or trip over my own feet it takes something that's already embarrassing and annoying and calls attention to it, making it more embarrassing. And I take pride in self-reliance, and the implication that I need assistance for the most absolutely insignificant of problems is insulting. I'd far prefer if people would just ignore my occasional (okay, frequent) clumsiness, or at least make it clear that they in no way expect that I need any sort of help by responding with "don't die" or "I hope you didn't scratch the paint" or the like.