A few months back I did a bunch of negative pull-ups [1] in an attempt to strengthen my pull-up muscles. The next day I was the most sore I've ever been in my life. Two days later the soreness increased. My arms were stuck in a 50 degree angle. I couldn't bend them straight or curl them up all the way. I had an intuition that something was wrong, checked the symptoms on Google, and saw something about a weird condition called rhabdomyolysis [2] where your muscles are so damaged that they start releasing damaging proteins into your blood that can damage your kidneys. Again, my intuition told me to go to the ER right away, which was unlike me because I'm your typical guy that usually trusts that my body will heal on its own. The doctors thought it was highly unlikely that I had rhabdo, but they ran the blood tests anyway, and were surprised to find out that I had diagnosed it correctly. I was hooked up to IV for 4 days after that, 24/7, pissing huge amounts of water every hour or so, because the only treatment is to just flush the proteins out of your system. If I hadn't gone to the ER that night, and had slept through it, I very well could have destroyed my kidneys.
At the ER the doctors told me that rhabdo cases were on the rise because of HIIT classes pushing people past their limits. From what I understand it's a rare condition, I think only 1-5% of the population at risk of it, but it doesn't hurt to know. The bigger lesson from this experience for me was to be more careful about pushing myself past my limits. I'm only 29 and I already see that my body can't take the beatings that I used to give it. Ironically I'm exercising every day now but am much more mindful of putting extreme strain on my body. I support high-intensity exercise, but I stop if I'm at the point where my muscles are exhausted and I no longer am doing the exercises with good form.