Random thought, since it's come up in the news and stuff again. People keep talking about double taps, and they're doing it wrong. A double tap is two shots on a single target in quick succession. It isn't banned by any conventions, it's often seen as a generally good idea, particularly with handguns (which are actually pretty terrible as far as guns go). Basically, if it's worth shooting once, it's worth shooting twice. The double tap has a big brother, the triple tap, which is just 3 quick shots.
What the press, people, and Zombieland keep referring to as the double tap, is actually known as Anchoring Rounds. It is the act of shooting someone on the ground, so the bullet finishes them off and "anchors" them down. This IS illegal, banned by military conventions, and is pretty much a terrible idea at all times (sticky situations happen against enemies who play dead then get up and attack, though. I believe there were problems with that in Iraq/Afghanistan for a while).