This isn't a nightmare. This is happening. The Mad Hatter is hurtling towards you with the likely intent of your kidnap and subsequent smuggling to Wonderland. Fortunately, you just so happen to have a vuvuzela on your person - so you blow it in his face and hope he goes away. Which he does; completing the marathon in a respectable four-and-a-half hours.
The above happened at today's 2013 London Marathon. As did the other stuff below, some of which is arguably more terrifying.