Had been cycling to uni next to the canal. Was about to overtake some lady. Lady decided to strafe to the right. My left had and/or handlebar hit her elbow. The next thing I remember is sky, the one after that - feeling cold. I was fine in the end. Phone died, laptop was incapacitated, but lives on till this day (shoutout to dell!). I had a broozed knee, although I can't recall hitting the gravel. Must've bounced off it into the canal. The bike, luckily, stayed ashore. Needless to say I was late for lectures that early september morning.
You can get a concussion without damaging your helmet.
I got a concussion in a car accident when some kids hit me: I was scratched up and bruised, but no injuries to my head - still had a concussion though.
Concussions come from your brain hitting the inside of the skull: Any change in movement of sufficient force might cause one.