A vital skill that is perhaps sometimes overlooked in some martial arts syllabi is First Aid. I recommend that everyone participating in martial arts classes obtain and maintain a First Aid and CPR certification equivalent to either American Heart Association or Red Cross standards. At the very minimum, these certifications should be prerequisite to black belt rank or teaching functions in a club.
Today I creased my skull working on putting up some vertical blinds for my wife. Classical concussion symptoms - that is, head trauma followed by change in mental status. In my case I was mildly confused and disoriented for a couple of hours after the six-foot long 1x6 board fell onto my head. I seem to have gotten over it - rested for a while, took a bath and washed the blood out of my hair, and HEY, the blinds look fabulous!
But seriously, concussion is a good thing to be able to recognize, because if it is severe (vomiting, amnesia, drowsiness, staggering, or crazy speech) you should seek medical attention. Repeated concussions, even mild ones, have been known to cause impairment disproportional to the apparent severity of the trauma. Children are particularly succeptible to sudden death from second or third concussions.
So, get some training, know when to seek medical attention, and know when to discontinue practice and get medical clearance to resume.