So, for all of half an hour between 22:00-23:00 GMT YouTube had a lovely 500 Server Error, with a 4150(ish) character hash on every page refresh. Streaming servers still worked, so all of those watching longer videos may not have noticed anything.
Everyone else saw this:
The text reads:
Sorry, something went wrong. A team of highly trained monkeys has been dispatched to deal with this situation. If you see them, show them this information:...some long hash
Source: © 2013 YouTube, LLC - www.youtube.com