So yesterday I ran Crota three times. The first time I was with a guy and he said that if you wait until the end of mission screen after defeating Crota you get a chance for an exotic. A couple people left, I didn't really believe him either, but I figured why not wait since I needed to make lunch. I hear "lucky f*ck" I look up and I had got the dragons breath. We ran Crota two more time and Atheon once, and nobody received drops at the end screen, so I guess the percentage is pretty low. But I can testify that it actually works. Edit: disregard this nonsense I guess.