Armed Altruism

The hero is alone in a Die Hard situation (with limited resources and running low on ammunition). He manages to free one or more hostages, and the first thing he does is to give away his only gun to one of them.

This may happen for several reasons. Possibly the rescuer is soon going to acquire another one himself. (You'd think, though, that in that situation, it would be better to wait until you actually had a second gun.) Possibly the rescuee is better trained to handle the weapon. Sometimes, however, it happens for absolutely no reason, other than to make things tougher for the hero and/or show what a good guy the hero is.

Note that this is completely different from any occasion where someone on a rescue mission gives a hostage a spare gun, as happens in Die Hard (which would have been a subversion anyway, as he deliberately gave the Big Bad an empty gun) and near the start of Mission Impossible III. This just makes good sense that both you and your ally/friend are armed while trying to escape.

Films

 * In the movie Air Force One, when James Marshall (Harrison Ford) frees the staff from a locked room aboard the aeroplane, the first thing he does is give his gun to the Secret Service man, which makes tremendous sense, regardless of his President Action abilities.

Gamebooks

 * Non-gun example: in the Lone Wolf book Fire on the Water, if you have the Magic Spear (the only weapon at that point that can harm a Helghast) you can give it to Lord-Lieutenant Rhygar so that he can guard the entrance of the Tarnalin tunnel. In early editions, you get killed for doing this.

Live-Action TV

 * In The Good Guys pilot episode.
 * In the Stargate SG-1 episode "Foothold", as soon as Jack O'Neill frees Major Davis, he gives him his pistol.

Videogames

 * In Deus Ex, after you free Agent Gunter you have the option of giving him your pistol.
 * Another example where the guy you're arming is a lot more dangerous than you are. In fact he probably doesn't need the pistol to massacre the NSF troops.
 * True. He's a LOT deadlier, mostly because he's invincible. In fact, if you give him only a KNIFE, he states "That will be adequate" and (If you didn't kill them)proceeds to charge and kill every terrorist in the area with said knife!
 * Also, you should have several other weapons at this point - the pistol is the most versatile of your weapons, but by no means your only one.
 * Just be sure you don't hand him the one with a weapon mod on it; those are limited resource throughout the game, and you will never see it again.
 * A player who isn't careful with their weapons can find themselves in this situation in Deus Ex: Human Revolution before the first shootout with Belltower in the Alice Garden Pods, as Arie Van Bruggen begs for a weapon to defend himself. . Of course, depending on Adam's augmentations, a gun may not even be remotely necessary to escape unharmed, ultimately subverting the spirit of this trope.
 * In Dead Rising, it's better to give your guns to the survivors you rescue because as NPCs they will have unlimited ammo.
 * But if a survivor seems suicidal, don't do it.
 * It's also handy because bosses tend to go for you first, freeing up the survivors to pepper them with bullets.