Okay, I'm going to be "that person." (sorry in advance)
You can still sustain a severe concussion without loss of consciousness. I would argue that most people who sustain a concussion do not know they have one. My first concussion was fairly mild. I got back up as soon as they'd let me (they were concerned about an injured spine... there was no EMT there, so my coach tried her best), and it wasn't until five minutes later that I became nauseated. Onlookers were fond of saying it was "the adrenaline," when in reality I'd hit my head pretty hard. It took years to put two and two together.
My fifth concussion involved 20 minutes of memory loss, but I was able to sit up (but not stand) immediately. I responded with a thumbs up when I was asked if I was okay. I wasn't, but nobody knew it because I responded. No one walked up to check on me, just let me sit; after an undeterminable amount of time, I was able to get up and stumble to a nearby seat. A year later, I actually asked if I had really given the thumbs up, because I remember thinking I wanted to do it, but wasn't sure if I had succeeded. I should have been carted off to the ER immediately, because it was only ~3 months after brain surgery.
Symptoms can be as mild as a headache ("well, I hit my head, so...") or a general fuzzy feeling (not necessarily dizziness). I think it has been said, but head protection will not stop a concussion... only, potentially, a skull fracture. All but one of my concussions occurred while wearing a helmet.
I'm not a doctor, but if you hit your head, please give yourself a 15 minute break to wait for symptoms before proceeding back onto the ice. Some symptoms don't appear (or are not noticed) for 24 hours. If you feel not quite right, go home and do something mindless (bonus points if you can get someone else to drive you or take public transport). Try to avoid mentally taxing activities, like reading or studying. You need to let your brain rest. If you have blurred vision, or nausea, go to the ER. They can't DO anything for a concussion except monitor you and do a CT scan, but it's best to go. If you see someone hit their head and begin to vomit, call an ambulance immediately - this is a sign of a brain bleed. Same if they are unable to stand. Experts say to never move an unconscious person... always suspect the spine has been injured. It's very, very important to not hit the head again once it has been concussed. This can easily lead to death. Every time you sustain a concussion, you are x% more likely to sustain another. The percentage only goes up as you accumulate more head injuries.
I had to fight to go to the ER after one of my worst concussions. Nobody would take me to the hospital, but everyone offered to drive me home (or follow me). I had to call my parents an hour away to take me. It's the responsibility of those who witness the head injury to speak up for the injured and make sure they receive proper care or are okay to continue on. Typically, they are not okay to do that on their own. I'm guilty of pulling the "I'm totally okay" card (case in point: My thumbs up story). Many people don't know the signs of concussion, and can't recognize when someone has been injured; I really advocate for people educating themselves on the signs.
I'm sorry to be preachy (truly) but concussion awareness is one of my soapbox subjects. I've had 5, plus brain tumor, plus brain herniation. I take head injury very seriously, always.
To answer the original question, I haven't witnessed any head injuries on the ice. I've seen many related to horseback riding that would make your hair curl.