The new Smart Recipient feature adds two (extremely helpful!) functions to your Gmail account: First, it analyses your email history and suggests who you should be sending your message to, based on who you email the most.
Second, if Gmail thinks you unwittingly included the wrong guy in your "Send To" list, it'll spit out a clickable suggestion to let you replace "the wrong Bob". These features will start helping you get out of sticky situations soon, as it rolls out to all Gmail accounts over the next few days. [Gmail Blog]