At least 10 people, including five children, were critically injured after the transportable rotating drop tower came crashing 50 feet (15m) to the ground. 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…
Rocket, Southport Pleasureland, Merseyside, UK
Poogee Penguins Spin Out Coaster, Santa’s Village, NH
A 51-year-old ride operator was seriously injured after falling 6-8 feet (1.8-2.5m) to the ground from the SBF Visa Group spinning coaster’s platform. Source
El Toro, Six Flags Great Adventure, Jackson, New Jersey
Thirteen people were injured, five of which were taken to hospital, when the roller coaster train suddenly jarred, as though it had hit a pot-hole, just before returning to the station. The attraction closed for the remainder of the season. Source
Train, Land of Make Believe, Hope Township, New Jersey
A 2-year-old child was seriously injured and airlifted to hospital in critical condition after being struck by the park’s train. Source
Feuerdrache, Legoland Deutschland, Günzburg, Germany
Europe Flyer, Venray Kermis, Netherlands
Eight people sustained minor injuries while riding the swing carousel at the street fair when they struck tree branches. Occupants may have grabbed the branches while riding, causing tension straps that were holding the branches away from the ride to loosen. Source (in Dutch)
Shipwrecked, Blackgang Chine, Isle of Wight, UK
Heisse Fahrt, Freizeitpark Klotten, Germany
A 57-year-old woman fell 26 feet (8m) to her death from the moving Gerstlauer Bobsled coaster when she slipped from her seat. It is believed that her lapbar was not securely fastened. Source

Body Count, Pencester Gardens Funfair, Dover, UK
A 14-year-old boy died after breaking into the funfair with some friends the day before it was scheduled to open. The teenager tried to climb the closed attraction only to lose his footing and fall to his death when he became trapped between ride equipment. Source