A squib load isn't exactly common, but it's not so rare that you shouldn't know how to recognize one, even if you only shoot factory ammo.
A squib happens when a cartridge doesn't have a sufficient propellant load to push the bullet out of the barrel. This can also occur if the propellant doesn't fully ignite. The bullet then becomes lodged in the chamber or barrel.
The trouble comes if you don't notice exactly what happened, assume it was a misfire, chamber a new round and fire. Then you have a full charge firing into a barrel obstructed by a bullet. This can force the first bullet out of the muzzle, or it can blow up your gun.
In this Gun Science installment, we teach you how to recognize the sounds and feel of a squib load, and what to do about it if you run into one on the range.