Failwhale is the Twitter error page
Twitter was regularly offline in the past by a too large amount of users who would like to use the service at the same time. The page you saw then was a whale that was lifted by birds from the water: This is the Failwhale. Meanwhile, many improvements have been made to the platform, but it still occurs that the servers are out (especially during upgrades or major events). The Failwhale has become a household name and is also often used if another site doesn't do it anymore. Failwhale images from Flickr