Sometimes I send messages I shouldn't send. Like the time I told that girl I had a crush on her over text message. Or the time I sent that late night email to my ex-girlfriend that we should get back together. Gmail can't always prevent you from sending messages you might later regret, but today we're launching a new Labs feature I wrote called Mail Goggles which may help.
When you enable Mail Goggles, it will check that you're really sure you want to send that late night Friday email. And what better way to check than by making you solve a few simple math problems after you click send to verify you're in the right state of mind?
By default, Mail Goggles is only active late night on the weekend as that is the time you're most likely to need it.
--Gmail Blog on preventing drunk e-mailing (HT: Chris Blattman's Blog)