I bet I'm not the only one who does this. A lot of times I am thinking a game is using the old B/X or 1e rules, but something was actually changed in the current OSR rules.
Case in point: I was thinking of creating an alternate vampire for BFRPG that required them to bite a victim to drain a level, because I thought that just touching someone to drain a level made no sense, based on all the books and movies. Then I reread the vampire entry in BFRPG and found that Chris had already done just that.
I didn't even know this, as I had just skimmed the monsters in the book. They had looked similar enough that I was just adding in old rules from memory to fill in the blanks of what I hadn't read.