Rule No. 1 states that all guns are loaded.
The Evyl Robot Soapbox blog has a very good story about a relative's negligent discharge. Fortunately, no one was injured.
The pictures illustrate why it is vital to know what is behind your target. Some of the 00 buckshot traveled across the room, through a door, across the garage, through an exterior wall, and embedded themselves into an outbuilding.
Dry fire is excellent practice. But you have to make sure the firearm is unloaded and all live ammunition is out of the room. Brownells (among others) sells a full range of dummy ammo and snap caps.