If you visit the Gmail > Settings > Lab page you'll find a brand new lab feature called "Got the wrong Bob?"
When's the last time you got an email from a stranger asking, "Are you sure you meant to send this to me?" and promptly realized that you didn't? Sometimes these little mistakes are actually quite painful. Hate mail about your boss to your boss? Personal info to some random guy named Bob instead of Bob the HR rep? Doh!
"Got the wrong Bob?" is a new Labs feature aimed at sparing you this kind of embarrassment. Turn it on from the Labs tab under Gmail Settings, and based on the groups of people you email most often, Gmail will try to identify when you've accidentally included the wrong person — before it's too late.