In a blog post, Google explained that if you normally email Bob Smith together with Tim and Angela, but happen to add Bob Jones instead, you'll be warned. While the feature only works if you're emailing more than two contacts at once, it can be paired with other embarrassment-saving Gmail Labs features like Undo Send to prevent email mishaps.
Enabling the new "Got the wrong Bob?" Labs item is a piece of cake. Log in to your Gmail account, click Settings at the top right of your screen, click on the Labs section, and toggle Enable next to the feature (which is located a little over 3/4 down the page). Be sure to check out the other Gmail Labs features while you're there, or take a look at our favorites here.