Broken right femur and torn MCL in a high school wrestling match at the end of my sophomore season. I attempted to back-trip my 265-pound opponent, caught my foot on the mat and his full weight came crashing down on the inside of my knee (I felt a pop and heard it; I was told that the pop was audible from several feet away according to spectators at matside). My femur broke at the base, just above the knee. Casted for 8 weeks, in a brace for another 8 weeks before I was allowed to put any weight on it.
The acute pain was blinding. I think I briefly blacked out, but I do recall involuntarily screaming and cursing for a few seconds, and then after I had started to gather myself, I remember saying something completely nonsensical to my coach about how I didn't think I could finish this match (it was the semifinals of a tournament), but maybe I would be able to wrestle for 3rd place later that day - even though I couldn't put a lick of weight on my broken leg. The pain subsided to a dull ache after 10 or 15 minutes, but it throbbed periodically for the next few days until I got it fully immobilized and casted.
Fortunately, I made a full recovery, and the opponent I was wrestling at the time became a friendly adversary. We wrestled again three or four times the following season; we also played football against one another. Fierce competitor but also an incredibly nice guy, he went on to play Division I football on scholarship and had a few tryouts for professional teams.