when your dive buddy invites you on a spear fishing trip and only wants you there to hold the stringer. when your buddy hands you a can of petroleum based lubricant to get that stuck O2 bottle valve to open. when after your first deep dive with the new reg set up you bought from your buddy, he turns to you and says, "im glad it works for you, it always stopped breathing for me." after a 30 minute dive to 120 feet, your buddy says, Oh, you wanted Nitrox? when 40 miles off shore, your buddy says, I thought you gassed up the boat?