I cannot STAND when people make obvious spelling errors, and to a lesser extent, grammatical errors (and I mean where you actually mispell it, not typing too fast). Or when people repeat some falsehood as fact.
when I correct people, I explain it and give some sort of history of whatever the error was, and half the time the response is, wow that is really interesting--because I'm usually tactful and polite and don't go overboard, and people are generally amazed that some person actually knows about the subject.
But then sometimes I don't know a person too well and they make some stupid error that drives me crazy. Last night a guy I don't know too well was at the pub with me and some people and he pronounced the word "ladle" (LAY-DUL), (the thing you scoop soup with) as "LA-DUL" (the LA being like "lap" without the P. I let it go and then he said it twice more and I said, "you mean lay-dul?" he didn't respond. He was just one of those people that grate on me anyway.
I've gotten to the point where I don't care about alienating people. I care more about truth than friendship, because any friend of mine will also value truth more--within reason of course, there is no reason to be rude. But some people propogate the worst error. You do need to find a balance. I think a good rule is that if you find you can't correct someone without alienating them, you should make a concious effort to not correct error. HOWEVER, if you are able to do it in a socially acceptable way, there's nothing wrong with doing it, just watch yourself--that's my thought, and what I try to do--sometimes I do go overboard, but the positive reactions I get generally outweigh the negative ones