Didn't Crosby keep playing after the hit by Hedman in the game where he was diagnosed with a concussion? Which was three days after the hit by Steckel in the Winter Classic? And didn't he look pretty fazed by that hit, like at the end of the game, not realizing that he should have skated onto the ice when his goalie came off? It seems like there's a decent chance he suffered a concussion then and just lied about it.
Didn't Ian Laperriere play in the preseason, lying to the Flyers about how he felt from taking those pucks in the face last year? And despite the fact that that he never recovered from the last one he took in the playoffs, he did come back and play with it in the playoffs too.
Last year a poll conducted by the AP showed that almost 20% of the NFL players surveyed had hidden concussion symptoms from their team. Before last year (I believe) players could return to a game as long as they hadn't lost consciousness. It was only a month ago that they established a standardized test for trainers or doctors to use to determine if a player might have a concussion.
Aaron Rodgers missed one game after he suffered a concussion against Detroit, but he didn't miss one after the concussion against Washington, earlier in the year.
In the NBA this year, Luc Richard Mbah a Moute of Milwaukee didn't miss a single game after suffering a concussion. He played three days later.
Players get concussed and come back days later (or just never leave the game, because the concussions aren't officially diagnosed) much more frequently than what you're suggesting.
Whats the common theme of what your stating? Either they were unaware of their injuries or they hid them from their team. In a professional fight BOTH fighters are checked for PCS and dilated pupils. I am sure a fighter would try to get away with it if they could but chances are they won't, especially with the athletic commissions checking up on fighters before, immediately after and shortly after a fight.