First, here's a squib load that occurred in a revolver of unknown make. This picture was taken by a gunsmith following repair:
Supposedly, the gun is still in good condition.
Some explanation: Squib loads typically occur when a round containing no powder is discharged, although no ignition or incomplete ignition of the powder can occur as well, it's thankfully much less common. Since the primer alone has enough power to move the bullet but not enough to force it through the rifling of the barrel, the bullet becomes lodged. This is the squib; subsequent firings cause the next rounds to impact the stuck bullet, cause a pressure spike, and usually bulge or shred the barrel, blow up the action, or cause severe damage to the gun (and possibly the operator). The only thing you can hope for when firing a squib is to have another squib!
From the picture above, the individual who fired the gun may not have noticed the lack of recoil or target impact. He's lucky.
Now, what happens when you have a squib in a 9mm submachine gun? This.
For those of you who like to shoot, just remember to be careful out there. If you fire and the gun just doesn't feel right, unload it, clear the action, and check that the barrel is clear!