The reason that I find this error so annoying in English teachers and students at elite colleges is that they're trying to sound educated without really being educated. These people look down their noses at those with less formal education, yet their speech reveals that they didn't benefit from the education that they're so proud of.
Don't worry about making an error in your posts; posts are often wonderfully spontaneous. Anyhow, I can't nitpick because I'm the queen of typos and misspell a word every now and then. Sometimes I forget some of the trickier punctuation rules-most of us do. We're not writing a term paper here.
I never said that I was perfect-or claim to be an English professor!