Another big one in our culture is location prestige. When I was reading a lot of scientific papers I lost count of how many times top universities on the coasts got credit for things that had shown up a decade earlier in some little known paper published by someone in some smaller midwestern or southern college. I saw lots of this in theoretical biology, math, and CS.

I think it was only rarely plagiarism. Mostly it was just that stuff only gets noticed when it gets published by a prestigious university lab. They were probably honestly unaware that anyone else had ever done anything similar.