Yes booms can kill and we should always inform all on board when making maneuvers, particularly down wind when jibing. Before jibing inform all on board of your intention to jib by yelling "prepare to jibe", make sure all on board inform you that they know what you are about to do, they should yell back "ready to jib." Then after they respond you yell, "Jibing." Once through the jib of course trim up and sail your course.
Sometimes when people are sailing directly down wind the wind can accidentally get behind the sail and throw it across to the other side of the boat, this is called an "Accidental Jibe." The wind should never "accidentally" get behind the sail. The sail will give you signs its about to come across accidentally via the boom lifting up (if boomvang is not tight, if you have one), or sail gets loose looking/baggy, has bad shape. Sails that are fully battened are harder to see lack of shape. Thus we should also be watching the wind as it ripples across the water, make sure the wind is hitting the sail at the correct angle.
Wearing a water sports helmet is not a bad idea to protect ones head. Hobie Cats roto-molded sailboats have no booms, which is a great safety feature particularly for new sailors. The advantage of a boom however does allow for better mainsail shape control for better performance.
Sadly some lessons are learned the hard way, so let us all learn from this and keep an eye on the boom.
Here is a nice video to show how to safely jib a keelboat. Shortly I will post some new videos myself of how to jib a small monohull and catamaran.