1 GM: You do realise you're having this discussion with a slavering alien monster about to attack and eat you?
2 Serron: {with the xenomorph still lurkng behind him} Oh... right... yeah...
3 [sound]: {Serron turns and fires a blaster at the alien} BLAM!!! {the alien collapses in a pool of green blood}
4 Serron: {calmly} NPCs never seem to take full advantage of attacks of opportunity.
4 Spanners: Indeed.

Well that solves that problem.

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2014-07-09 Rerun commentary: An attack of opportunity is a rule in some roleplaying games, particularly Dungeons & Dragons and related games: it allows combatants who would not normally be entitled to attack a particular defender to attack that defender if the defender chooses to take an action that effectively lowers their guard.

For example, if you turn away from an enemy to flee from the combat, the enemy may get an attack of opportunity to strike you as you turn away, but before you get out of range. There are other possibilities too, depending on the game system. Doing anything other than actively being on your guard may provoke an attack of opportunity - say taking a few seconds to rummage through your pack to find a potion or whatever.

Reading this annotation could be distracting too. If a goblin had been lurking behind you right now, you'd have been hacked to pieces by the time you finished reading this.