This one is a bit unlike the others, since it hasn't been introduced and then forgotten - instead it's been introduced, forgotten, reintroduced, forgotten again, etc. In "Cathexis," Tuvok has to stun a number of people in only a split second, a feat he achieves by using the phaser's "wide beam" setting, of course, which was never actually introduced as a new feature that was only available to Tuvok. This phaser setting can stun (or kill) a number of opponents over a wide area with one shot, but it has frequently been forgotten when the episode demands it. For example, in "The Siege of AR-558," a group of Starfleet officers armed with phasers protect a fixed position against a Dominion assault. Rather than bombard the enemy with a few phaser shots on the wide beam setting, however, the battle is fought much more traditionally, one shot at a time. If only someone had remembered the wide beam setting, the casualties would have been much lighter.