The importance of proofreading cannot be overemphasized. I see mistakes in advertising that's running all the time, and I'm talking about unintentional mistakes- not intentional misspellings like the Chick-fil-A billboards (which I love). but you'd think they'd raise the bar for signs that are supposed to guide public behavior.
In a recent Christmas trip to Baltimore, I saw this sign on the side of a grocery store. Walking by, it caught my attention at first because of the handwriting on it...handwriting proclaiming how amazing it is to misspell a word on a public sign that's only two words long. I wonder if the sign affects the enforceability of the law. After all, the sign says "NO LOITERIING," not "NO LOITERING."