Unlike some of Bush's earlier misstatements, this one was supposed to be a joke.
At the 2008 G8 summit in Japan, President Bush attended a private meeting with other world leaders to discuss carbon emissions. According to news reports, he concluded the meeting his last as U.S. Commander in Chief by saying, "Goodbye from the world's biggest polluter." Then he reportedly punched the air and grinned while Britain's Gordon Brown and France's Nicolas Sarkozy stared at him in shock.