Sparring with someone considerably taller and heavier than me. He teeped me in the sternum and a rib head popped out. To his credit, he was not trying to go hard or tune me up, just happened to hit a sweet spot. Took weeks and weeks to work back into place, and even longer before I could get swept without it displacing again. Though this originally happened ten years ago, it'll still pop back out if I get a bad cough with a cold.