I was fortunate when at school to have had teachers who had the time and inclination to educate us in related and sensible matters that were off-syllabus. In those days safety aspects were never discussed beyond the obvious common sense of "chisels are sharp. Use with care." This note was first prompted by a warning and exlanation, given to me in the carpentry workshop.
In the carpentry workshop one day, whilst waiting around for someone to finish with something, I was idly, although gently, swinging two hammer heads against each other. They bounced, and made an unusal, interesting, ringing sound in addition to the expected clunk of two heavy-ish objects coming to a sudden stop.
The steel face of the hammer is especially hardened, to enable it to take heavy use without being dented. I assume some of the extreme heat energy and subsequent quenching in cold oil is locked into the metal by the bonds that bind the atoms together in the complex compound that makes for the hardened iron that we call steel. Hammer heads take particularly hard wear ~ and so need to be very hard.
Nails ~ and other items that are normally hammered ~ are softer than the hammer head. So the nail takes all the wear ~ the hammer head just bounces off after driving the nail into wood, or whatever. If ane hammer head hits another hammer head then two very hard surfaces collide. The hardness also causes a brittleness, and if this occurs near the edge of one of the hammers there is sufficient energy for a flake of metal to fly away. As it does so some of the pent-up energy causes it to move with great speed. This flying chip can be likened to exploded shrapnel ~ and it cn do similar damage.
This seems unlikely or even impossible from such an item used for bashing nails and everything under the sun. Be assured ~ it does happen. What is more worrying is that sometimes the human body gets in the way. It does not have to be two hammer-heads. There are parts of machines that are made of very hard steel. An engineer will know about this ~ as part of the long apprenticeship ~ and will avoid hitting such things. They have devised other methods to cope with the need to bash two hardened surfaces ~ browse online for Engineer's Pullers
I suspect that, even in this health and safety conscious age some people may be unaware of an extreme danger. It is very unlikely to happen ~ but it does. An important stimulus for writing this cautionary note is because I have witnessed the tragedy of a recent encounter from our very experienced domestic repair tradesman. He called one day to fix a washing machine, and had very dark glasses ~ even indoors. He happily explained. Instead of using pullers he ~ gently ~ tapped a tricky bearing. A tiny chip exploded out ~and lodged in his eye. After several painful operations ~ and a one-eyed couple of years ~ he now has partial eyesight restored in that one eye ~ the best that can be done ~ sufficient to help the better eye to work well enough for him to return to his trade.