swollen battery in iphone 3gs forced out the screen

After storing an iPhone 3GS for 6 months or so I attempted to extract some photos from the phone only to find the screen had pushed out of the body & the glass had separated from the screen. I removed the screen unit to find the PCB bowed & cracked, after removing the board the battery was clearly visible blown up like a balloon! As you can imagine I was pretty worried as to what might have happened had I not discovered it when I did. Would it have exploded or caught fire & as it was stored in the bottom of a wardrobe it would probably burnt the house down. Is this a common problem or am I in the minority. it says on the back of the battery that there is a possibility of catching fire but that's no help when it is buried within the phone out of sight.