Rule 7: Do not take the piss -- Rules and Regulations, Profound Decisions
http://www.profounddecisions.co.uk/misc/rulesandregulations.asp
There's no legislating for stupidity or immaturity. Or is there?
Within UK Live Roleplay, Rule 7 has become one of those rules that usually goes unwritten - do not be a moron; act within the spirit of the game, rather than the letter; do not be an anti-social idiot. Only, this time, it has been written down, the quote above is taken directly from the small print of the booking form for Maelstrom (a game run by Profound Decisions), and is also a rule for the Pagga message boards.
Harry 'H.' Harrold states that he was the first to call it "Rule 7", in reference to use on the internet (possibly the Lorien Trust's Ley Nexus), and had previously used it as a first rule in a homespun game in the 1990's.
It's also been used as an in-character law by the Vipers in Lorien Trust, but as their third law, after Viper don't do Viper [sic] and Don't get caught.
All in all, it's a good maxim to live your life by.
References
http://www.pagga.net/cgi-bin/vBulletin_v3//showthread.php?t=12875