Gmail Labs' new "Got the Wrong Bob" feature saves you the embarrassment of sending an email to the wrong contact by alerting you whenever you compose an email to a contact with the same name as any of your frequently emailed contacts.
Got the Wrong Bob joins the forces of other embarrassment savers available in Gmail Labs, from the previously mentioned Undo Send feature to the silly but useful Mail Goggles feature.
To enable any of these Labs features, just head to the Labs settings page, enable the feature you want, then save your changes. Hopefully in the future you won't get another "Did you mean to send this to another Bob?" response from a wayward contact.
New in Labs: Got the wrong Bob? [Official Gmail Blog]