There is a hidden danger in 9 volt alkaline batteries. They tend to explode if they are shorted. A short occurs when anything metallic comes in contact with both terminals and the battery overheats. The following picture is a standard 9 volt alkaline battery that was sitting in a tray on the console of my Chevy van.
Apparently the battery came into contact with a keychain or other metal object in the console tray. I must have turned a corner and it shorted and within a minute or two there was an explosion on the console. Parts and debris flew around the driver's compartment.
From the picture it looks like one of the batteries cells center electrode actually protruded out the bottom of the case and caused other debris to be displaced.
New batteries should have a plastic protective clip on the terminals to prevent shorting. The Eveready Industrial batteries sold in a carton have this plastic clip on their product. I don't believe any Duracells come with this protective measure.
Two anecdotal stories come to mind:
I knew a sound technician that kept a few batteries in his pocket so when a battery in one of his wireless microphones became week, he'd replace it. One time two of the batteries' contacts shorted out and burned a hole in his pocket and left a scar on his leg.
In another occurrence a friend had left a few batteries on the mantle in his living room. His cat happened to crawl up on the mantle and inadvertently pushed a battery against another metal object. In a few minutes there was a large pop and the cat went flying across the room.
It's interesting to note that there is no warning on packages of batteries exploding with the exception of disposing in a fire.