This is a biggy. I do it all the time, particularly at scheduled events. You rush to your next game, get all your stuff straight, pore over the scenario rules, roll to see who picks/places and off you go. Then, on turn 3, it happens: Guy 1: "Uh I get a 4+ cover save over here..."
Guy 2: "Wait, 4+? I've been giving myself 5+ all game in these ruins. They are about the same..."
Guy 1: "Well the rules say X."
Guy 2: <flips table over and rips his shirt off, running out the door screaming profanities>
Well, I've never seen that last bit happen, but ask yourself how many times you've screwed yourself over by assuming something about terrain only to realize that your opponent has been assuming something entirely different?
While this is not usually a big deal (typically only a one-place difference on the assumed cover save value), it happens so frequently, in nearly every game I play, that I expect it's had a pretty big overall impact on my game.