The link shortening and redirection service TinyURL went down apparently for hours last night (it's still down, in fact), rendering countless links broken across the web. Complaints have been particularly loud on Twitter, where long links are automatically turned to TinyURLs and complaining is easy to do, but the service is widely used in emails and web pages as well. The site claims to service 1.6 billion hits each month.
The moral of the story, though, is that it isn't supposed to work this way. There ought not be one single point of failure that can so easily break such a big part of the web.