I'm pretty certain I had at least a couple of concussions playing rugby at school. There was one incident where my own teammate rammed my face with his head after we tried to tackle the same player who ducked out of the way (my teammate in an offside position, by the way!). I completely blacked out for several seconds - which doesn't sound like much but is several seconds too long. Fortunately, on that occasion, my game was ended due to the blood pouring from my flattened nose, but there have been other occasions where I just played on.
Same when my son was playing for our local club, which fills me with horror when I think back to it. They were taking part in a local tournament, with several mini games over the course of a single day. In one game, he got a knock and was clearly dazed enough to require sitting out the rest of the game, but the coach let him play the next game. I know now I should have refused to allow that, and indeed should have just taken him straight to hospital.
There was another occasion when he got tip-tackled and landed vertically on his head.
Kind of glad he doesn't play rugby any more.
Sad to see what has become of players like Thompson and Popham, but the most upsetting aspect is that it's entirely unsurprising with the benefit of hindsight.