was doing round robins, had my dude in my guard and he wrapped me up for a guillotine, I sat forward to alleviate the pressure so he couldn't get leverage. he leaned back to try to wrench my neck causing my butt to come off the ground since I was already sitting forward, then he for some reason dropped all his weight on top of my shoulders, slamming my butt on the mat and bending/compressing my upper spine really hard. my back went rat-a-tat-tat like a tommy gun and I made a nice groan/gurgle noise as the air was compressed out of me. it was a total accident, we both did something we didn't expect the other to do, and I don't think I'm hurt, but my upper back is stiff as shit. He felt REALLY bad though, it was obvious he wasn't trying to be rampage and guard slam me or anything, but he still felt real bad.