I remember seeing something like this on Cracked and the only one I can remember is "We do not steal from wishing wells."
It's one of those things you don't think about until it's pointed out to you. Unless you're really destitute (and maybe not even then), it's considered really bad luck or at least horribly immoral to steal from a wishing well (and not just ones marked as "the money here goes to charity"). Most people who are over the age of 8 do not really believe that tossing a quarter in a fountain is going to grant a wish, it's just a fun thing we do for a "sense of magic," I guess, and yet, it's common courtesy to leave it alone. Even if people don't always believe in their wishes, those coins represent wishes, so sticking your hand in to get the free money is something of a marker of "you are a horrible person."
One social rule that I don't always follow that frankly kind of annoys me is the "You must have a pot to pee in." That is, I was out by a creek today, out in the woods.... and a couple of times I had to look around for cover so as not to be seen by other fisher-people and to you know, not be arrested or gawked at. If I were a rabbit, no one would care. It's nature, and when you're out in nature, it should not be any big deal when nature calls. Maybe I just outed myself as a gross person, but you know, medical conditions and freaking WOODS. I'm sure you've done the same upon making sure there wasn't a cop around at least once in your life.
One rule that doesn't apply to me anymore because I am no longer a kid and do not have kids... Teaching your kids to believe in Santa Claus. This winter I watched the Nostalgia Critic take that on. He argued that there is a purpose to it... that it has to do with not only magic and wonderment, but in setting up children for disappointment so they can handle the bigger disappointments of adulthood. It wasn't even cynical, it was "dealing with the loss of belief in Santa Claus is a natural thing, and while devastating to a lot of kids, helps them to deal with the hard spots of being a grown-up down the road." However, not all parents tell their kids to believe in Santa. There's a minority of parents who don't want to lie to their kids (often for religious reasons or conversely, anti-religious reasons), but for some, it's just an honesty-thing, and see how quickly everyone hates them for it! It's like "You don't let your kids have the fun of believing in Santa! Gasp!" and it doesn't seem to matter what reason, even "I don't like lying to my kids."