Death on Big Thunder Mountain: 22-year-old visitor Marcelo Torres died on September 5, 2003 after suffering severe blunt force trauma and extensive internal bleeding in a catastrophic derailment of the Big Thunder Mountain Railroad. Ten other riders were also injured, and the cause of the accident was determined to be improper maintenance. Torres’s family won a substantial settlement from Disney several years later.
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.