"Don't forget Bob" and "Got the wrong Bob?" Gmail Labs features today graduated from Gmail Labs.
The features help prevent you from making two common mistakes: forgetting to include someone on an email, and sending a message to the wrong person with a similar name to the person you meant to email -- like emailing Bob (your boss) instead of Bob (your friend).
Similarly, if you click on a suggestion to replace a mistakenly added recipient, the proverbial "wrong Bob" will be replaced by the right one.
Gmail begin turning them on for everyone (they should start working in all Gmail accounts over the next day or so). Once that happens, as you type in your recipients, Gmail will automatically make suggestions based on the groups of people you email most often. When you see a suggestion to add a person you've forgotten, all you have to do is click on their name to add them.
[Source: Gmail blog]