One of my phones from a few years back had no cover over the battery pack. The back of the LiPo battery case acted as the "cover" so it was fairly exposed. One day I noticed that I had my keys in the same pocket and that I must have sat down with the keys pushing against the phone. There was a dent on one side of the battery pack and a little bump on the other side where it must have deformed every layer inside the battery. I was lucky that it didn't explode or burst into flames in my pocket!