1984 Matterhorn Death: On January 3, 1984, Dolly Young was killed when she was thrown from her Matterhorn Bobsled car, and struck by the next bobsled. A subsequent investigation found that her seat belt was not buckled, but because she was riding alone, it was never clear if she deliberately unfastened her belt or if it malfunctioned.
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.