From www.wdwinfo.com

Headless Horseman falls at Walt Disney World's Halloween Party
Oct 11, 2012

One of the most popular attractions at Mickey's Not-So-Scary Halloween Party at Walt Disney World is the Headless Horseman and his galloping horse before the "Boo to You" Parade. On Monday, October 8, the horse fell while going through the Liberty Square area of the Magic Kingdom. The rider fell off the horse, but quickly got to his feet and was later said to be doing ok. A series of videos on YouTube shows the horse continuing on without his rider and then the Headless Horseman walking along behind. He is later seen remounting the horse to complete the parade.

The horse and rider did not ride in the party's second parade, due to medical evaluations taking place. Disney says they don't believe the weather played a part in the incident, though it did rain earlier that day. The Headless Horseman was set to ride again at the next Party.