I think it was pine tar. But seriously, it's not that major of a thing. It's one of those rules that are in the books that everyone usually ignores--except when they need excuses. It's like that pine tar rule with the bat--recall that guy who hit a homerun and then they called him out because the pine tar on his bat was wider than home plate? Dumb rule. See the article linked in my post 153.
that would have been Geroge Brett. I think it was Billy Martin who called him on it.