A friend of mine was solo backpacking in Alaska (Denali National Park). As per sensible, he had his mega can of Bear-b-Gone (ok, I made up the name, but it's OC spray with 10% active ingredient) clipped to his shoulder strap for easy grabbing.

Somehow, while taking off or putting on his pack, he managed to blast himself in the face from just inches away. He said the pain can't be quantified in English-- like a million bees with acid for venom stinging him was one way he tried to put it. The spray went directly up his nose, in his eyes, into his mouth, his ears... he stumbled in agony to a nearby creek and dunked his head into this glacial runoff stream for 2 hours until the swelling went down enough for him to see. Poor blighter!