Goddamn awfulness, that's what.
Above: Anatoli Bugorski, Russian scientist and victim of a particle accelerator accident
Oh you want specifics? Alright, so technically nobody has ever been so unlucky as to find themselves in the path of either of the Large Hadron Collider's proton beams while they were actually turned on, so the real answer is that nobody's really sure what would happen if you stared down the barrel of the world's highest-energy particle collider and pulled the trigger.
THAT BEING SAID: There once was a man unlucky enough to find himself in the way of a proton beam belonging to a much weaker particle accelerator. And while that man actually survived, the extent of his injuries suggest that a similar encounter with the vastly more powerful LHC would almost certainly end very, very poorly.
The only precedent scientists have for a collider injury occurred in 1978, when a 36-year-old researcher named Anatoli Bugorski [pictured up top] managed to get his head in the way of the proton beam in the U-70 synchrotron in Russia. (That machine was just one hundredth as powerful as the LHC.) According to journalist Masha Gessen, who interviewed Bugorski for a 1997 article, the zap burned a hole from the back of his head to just beside his left nostril and left him with facial paralysis and epilepsy. (There's no indication of the exact shape of the hole.) Remarkably, Bugorski was able to continue working as a scientist after recovering from the initial trauma.
Just imagine it: trepanation and/or decapitation, mediated by a particle accelerator – what a way to go!