There is a lot of subjective advice out there and one should always way out their references, but here is a set in stone piece for all: please never use your axe or hatchet's poll as a sledge, hammer
or wedge (when one uses another sledge to pound the back of the axe through).
mushrooming is a symptom of pounding on the back of a hatchet or axe.
the exception to this rule are axes/hatchets that have special tempered polls made for striking.
unfortunately, the vast majority of polls are not tempered to give or take impact.
not only does the impact show on the curled burr that forms but also denting on the poll surface itself.
the good news is that mushrooming does not mean the end of the tool, although it is a serious red flag.
if there is evident mushrooming then the eye must be checked for
deforming (sagging on one side or the other) or cracks coming from the eye.
if these are there then the can no longer be trusted as functional.
if there are no other defects then the mushroom has to be controlled with filing.
it pains me to do this because it is a reminder to poor use/abuse, but other than that the tool is still in perfect to use.
getting the tool back to work is a way to reverse its history and start afresh.