I'm in the unfortunate position of having had two concussions in my 4.5 years training.
The first was about 3 years ago when someone was I was thrown doing iriminage and someone else was thrown onto my head - really shows the importance of awareness and throwing towards the outside of the mat. That one was really bad and I had nearly a month of work with chronic headaches, memory loss, concentration difficulties, etc. Not to mention the fact that I can't even remember my 4th kyu test which I sat a few days after getting my clearance!!! What was really annoying was the other guy had "a slight headache". Three years later I still have problems - migraines and concentration difficulties, especially when I'm tired.
The second occurred when we were doing ushiro ukemi practice and someone was pushed out of the way of another student and straight into my head. Not so severe this time thank goodness.
Both of these injuries could have been avoided if people were more aware on the mat. To my knowledge I am the only person in my dojo to have suffered a concussion, let alone two.