I screwed myself a few years back. Had a client with a very sweet older black lab that unfortunately had a multitude of health issues in his old age. Half deaf, arthritis, etc. But I was happy to look after him, because for the most part, they were manageable issues.
Well, one day I show up, and it must have been a bad day for the dog, because he was simply not able to stand up. Unfortunately, the only way to get down to the client's yard for the dog to do his business was down a flight of stairs. After trying every trick in the book to get him up, I finally decided I'd have to carry him down the stairs myself.
Healthwise, probably the biggest mistake of my life. Hurt my back, and had interspersed sharp, shooting leg pain over the following weeks. Worst pain I've ever felt. Finally got to a doctor, and he guessed what probably happened is the sciatic nerve got wedged between two spinal columns, and every time I moved, it'd pinch that nerve, causing the pain.
It's gotten better as the years have gone by, but every once in a while, I'll move the wrong way and it's like a lightning bolt of pain just rips down my leg.