Junior year football. We were running a pass rushing drill, with an oversized, heavy-as-hell tackling dummy affectionately named "Big Bertha" playing the role of QB. I was going up against clearly the biggest guy on the team, who had some quickness as well. He was playing OT, I was playing DE. I get off the line much quicker than I usually do and actually beat him around the corner. However, I didn't quite clear him and I was a little shallow on depth. As a result I was going to miss Big Bertha. Because I couldn't let go of my pride of beating the the biggest man on the team, it was then that I pulled my dumbest stunt ever in football - I decided to arm tackle Big Bertha. I stick my arm out and Bertha hits me right on the inside of my right elbow and pops my shoulder right out. Right then the coach blows the whistle to move on to the next drill, and I'm left to lie on the ground writhing in pain for a good 3-4 minutes until coach realizes I'm not lined up for the next drill and gets ready to completely chew my arse out until he realizes I'm not okay.
One other one (just to show you what a bunch of d-bags I played with) - we were running an onside kick drill. Ball is kicked straight at me, I recover it, hit the ground, and cradle it. Dead ball (or at least it should have been). About 3 seconds later I get absolutely DRILLED by 3 of the biggest guys on the team - one helmet to the kidneys, one in my back, one to my helmet. I was woozy for the rest of the practice.