Space Mountain Death: On August 14, 1979, a 31-year-old woman became ill after riding Space Mountain, and she was unable to exit the vehicle. Although employees told her to stay seated while the vehicle was removed from the track, other ride operators did not realize that her vehicle was supposed to be removed and sent it through a second time. She was semi-conscious after the second ride, went into a coma and died one week later. The coroner's report attributed the death to natural causes and a subsequent lawsuit against the park was dismissed.
Seriously though I am surprised deaths don't happen more often. The Walt Disney parks had 132,549,000 visitors just in 2013. So just in that year alone the average was more than 37 thousand people per day.
I'll bet you it is statistically safer to play and have fun inside their parks than the trip flying or driving there.