OK, it wasn't me who experienced it, but I was there to witness this... Put me off meat for a while...
Me and some friends were going to KFC. Max ($andy Mudukon) ordered a chicken burger, he bit down and the burger appeared to explode with some sort of yellow, greasy substance. We instantly assumed it was puss (I'm not saying it really was), and I joked about him biting down on a vein.
Man, he hasn't eaten there in years.