I just finished up fly-sparging an 11gal batch of Munich Helles and had the wort in the kettle on the burner. I took the time to clean out my new cooler mash tun while I was waiting to reach boil. I had a bucket lined with a trash bag for the grain. After all the grain was out of the mash tun I tied up the bag after pushing most of the air out to save space in the trash can. I then pick up the bag which was now tied off and full of hot grain. When I threw it into the can the whole bag exploded in my face spraying spent grain everywhere including the ceiling of my garage. My guess is the hot grain weakened the bag plus caused an air pressure increase making the bag a big grain balloon.
Moral of the story: Let the grain cool before throwing out the bag.