Repeated hammer blows against a wooden or metal shaft, pole, stake, handle, or other similarly shaped object can result in a specific type of deformation called mushrooming. Mushrooming is so called because the resulting shape resembles the familiar cap of a mushroom fungus sporophore.
When a hammer strikes the end of a shaft, a great deal of force is transferred through the shaft to whatever object is at the other end of it. The transfer of force is not perfect, and some of the force is absorbed by the shaft as it begins to deform at the impact site. Since the rigid shaft is strongest in the direct line of force, very little deformation can occur in this direction. Instead, the shaft is deformed radially outward as the material spreads in any direction it can. The result is that the end of the shaft develops a shape not unlike the top of a mushroom.
In many cases (chisels, fence posts) this deformation is normal and expected, and ultimately doesn't hurt the functionality of the shaft in question. However when dealing a bolt, screw, pin, or other fastener which is intended to fit through a hole, mushrooming must be avoided or the fastener will no longer be able to be inserted or removed. Often this is forgotten when a stubborn or rusty bolt just won't pull out or a bearing won't come off its shaft, and an attempt is made to force it with a hammer. The resulting mushrooming will only make the problem worse as the expanded section will no longer fit through the hole at all. Mushrooming will also expand and deform the threads on bolts and threaded rods, rendering them useless for screwing into a nut or tapped hole.
Mushrooming can be removed by grinding the expanded end back down to its original diameter. Tools are more resistant to mushrooming if the dumb end is beveled.
This feature of mushrooming can of course be put to an advantage as well, for example as a cheap and permanent method of securing a metal label to something. First, two pins made of a soft metal are set in the object to be labeled. Then two holes are drilled in the label and set over the soft pins. By mushrooming the pins over the holes with a hammer, the label can be set in place permanently - the mushroomed ends of the pins are now too wide for the holes in the label.