Killing Bow
In storytelling, a plot element set-up for future payoff is known as “Chekov’s Gun”. The idea being that you don’t introduce a gun in Act I if you don’t intent to fire it by Act III. Magic items in D&D can be similar. As a DM, you shouldn’t introduce a cool magic item that you don’t want to be used or abused. And sometimes it’s fun to set-up a future encounter via some neat treasure. But this can’t always be planned for, and the players really might forget that they have the perfect magic item in a situation until the moment has passed.
So right. Learned this the hard way. Including that time I gave the party a potion of cure poison at mid-campaign, and by the time they got to the duke, who just got poisoned in a treason attempt, the (low level) cleric of the group totally forgot about that. Well, the party’s paladin got to marry the duke’s daughter and became a just and noble ruler…
My brother misread the section on arrows of ____ slaying. He got in in his head that if you fired the arrow, it would automatically hit one of the targeted creature, regardless of whether or not there had been one there before you fired the arrow. We kept his version at the table, and he’d just randomly fire off and sometimes kill a creature that nobody had noticed was there before. No XP, loot, or anything more than a single meal’s worth of meat from this sudden monster, but lots of fun was had at these arrows of induced random encounters.