"Wouldn't it blow up with the case going one way and the bullet going the other? You're saying it would just go ffffffffftt."
Yep, it pretty much would go ffffffft.
Smokeless powder is progressive burning, not explosive, Meaning that it burns faster and more efficiently the more pressure it's under.
In a gun you have a vessel that allows for pressure to develop rather nicely. The gun is essentially a pressure vessel.
If a round goes off uncontained, though, the casing itself is a pressure vessel. As pressure begins to rise as the powder burns, the bullet begins to move and pops out. Once the bullet, essentially the cork in the bottle, pops, pressure drops very rapidly.
The powder doesn't have the chance to develop the pressures it needs to burn efficiently or completely. Chances are if a round did go off in your pocket you'd find a lot of unburned powder and soot.
The biggest danger in a situation like this is the primer cup. If the primer compound pops, it is an explosion, and the primer cup and develop significant velocity. At short range it's more than enough to cause an injury at short range.