A park employee suffered serious injuries attempting to retrieve a mobile phone when he was struck by the coaster and a male passenger, who required surgery to save his little finger. Source
Accidents
Theme parks and roller coasters are statistically very, very, very safe. Out of the millions of people who ride the thousands of rides every year, there are only a small handful of incidents. You are statistically more likely to be involved in an accident driving to an amusement park than having one at a park. The rarity of accidents is one reason why the media report them, similar to plane crashes.
Every important element of a ride is inspected on a regular basis and every ride is tested every day without passengers before it opens. A ride might not open, or a safety system might activate, if there is an imperfection. For theme parks to operate, they must obtain insurance and one stipulation is their rides are frequently inspected and signed off by a strict Health & Safety governing body. Every theme park does what they can to prevent accidents because the negative press would be bad for business.
Having said all that, theme park accidents still happen…
The Smiler, Alton Towers, Staffordshire, UK
Four riders were seriously injured when two of the coaster’s trains collided into each other. A 17-year-old girl had to have her leg amputated above the left knee and a 20-year-old woman had her leg amputated below the right knee. Source
Hero, Flamingo Land, Yorkshire, UK
A 15-year-old girl and a woman, 26, were in the queue when a metal foot-rail became detached from the ride and struck the pair. The woman was treated onsite and the teenage girl was sent to hospital with head trauma. Source
Sizzler, Church Carnival, El Paso, Texas, USA
A 16-year-old girl was killed after striking a metal pole when she and another teen were ejected from the fast spinning scrambler ride. Source (with video report)
Flying Saucer, Temple Fair, Xinxiang, China
Nineteen riders were injured, one seriously, when the revolving arm of a giant frisbee ride disconnected from the supporting pillar. Source
Boomerang, Fantasilandia, Santiago, Chile
A coupling failure caused the train to separate during the backwards portion of the ride. Only minor injuries were reported. Source (in Spanish)
Sky Ride, Winter Wonderland, Cardiff, UK
High winds caused the seats on this traveling Star Flyer type ride to crash into each other. Nobody was hurt in the incident. Source
Grand National, Pleasure Beach, Blackpool, UK
A 58-year-old man was left paralyzed after breaking his neck during a ride on the historic wooden coaster. Source
The Ultimate, Lightwater Valley, North Yorkshire, UK
The coaster struck and decapitated a deer near the track covering riders in blood. No one was injured. Source
Spinning Barrels, Holiday Park, Rheinland-Pfalz, Germany
An 11-year-old girl fell onto the ride platform and was crushed to death. Source