SDC always has been one of the safest, if not the safest parks in the world, but stuff does go wrong every blue moon. Something like 20 years ago there was a gruesome incident where a rider on FITH was killed when the train he was in was accidentally diverted into the maintainence area. The entry into the area was only as high as the car, so when it went through he was decapitated. Since then they've done such a good job of keeping everything run as safely as possible that there have been no other incidents involving major rider injury chain-wide, at least to my knowledge. (Has anyone ever died or been injured badly at DW?)
Also, as I just learned recently, there was an incident with the bridge holding part of the Lost River's queue just a few years ago, when part of it collapsed. One woman broke some bones in her leg. It was apparently just one of those structural oddities that occur out of the blue.
Other than those two incidents, I don't think much else has happened, thankfully.