Basic Trope: You get an item or power that can only be used once, but it is so awesome that you hold onto it forever.
Straight: The BFG kills all the enemies in the room in one hit, but there's only one round for it in the whole game.
Exaggerated: When you fire the weapon, the game reminds you how rare it is and makes you confirm your choice multiple times.
Downplayed: The weapon has a recharge time that takes a while. Although it can be used more than once, players tend to only use it for the bosses to make sure the item recharges by the next boss fight.
The weapon was so powerful that the scientist who crafted it only created one round so as to control its power.
There is only one of those items which you'll need to defeat the True Final Boss.
"I TOLD YOU this thing was the real deal, you fool!"
A shopkeeper occasionally sells extra rounds for the weapon, but has a very slim chance of having them stocked.
Or they're VERY expensive.
Averted: The weapon has plenty of ammo scattered about the game to use it in all reasonable situations.
Enforced: The weapon is deemed broken. The level designers want to prevent you from strategically using the weapon to skip all the hardest boss battles, so the ammo must be limited.
Lampshaded: When you're given the gun, you're told that you only have it because all the previous heroes never wanted to use the one round.
The Big Bad allows the player to acquire only one shot for the weapon, knowing it will prevent him from ever using it.
Double points: The Big Bad does this to hide the ammo or keep it out of the hands of those who wouldn't hesitate to use it against him.
Exploited: The Big Bad chooses to give the hero plenty of powerful items that each can be used once, knowing that they will take up precious inventory space and deprive the hero of more mundane, but more useful potions.
Defied: The player uses the weapon.
"There's a difference between being prepared and being an idiot. That gun could save your ass. Don't hesitate to use it if you need it."
"It took us an arm and a leg to get him that gun. How much you wanna bet he's just gonna finish the whole mission without using it?" "More likely we'll have to go in and recover it from his dead body."
Conversed: "Did you ever actually use the Smart Bomb?"
The player character treats the weapon like a living thing, giving it a name and talking to it occasionally. Using or losing the weapon puts the character at risk of depression.
The character's refusal to use the item because he keeps thinking the situation's not dire enough eventually costs him his life.
The character accidentally drops the weapon into a lava pit, causing him to descend into depression. Later, after much emotional trauma, reflection, and gaining acceptance of the weapon's absence, the character is given another one and told to use it wisely.
The character is brought back to life and given a power boost by the sapient item, which reveals that it was made as a Secret Test of Character to see if he would try to go as far as possible on his own merits rather than try to tap on it at the first sign of trouble.
Played For Laughs: After taking about a week to calculate a formula to determine the cost-effectiveness of using the BFG vs. using weaker weapons, The Hero decides it's okay to use the BFG if he's ever faced with 200 Mooks in the same room. Sure enough, he opens a door and sees 200 Mooks in the next room. Then he shoots one of them with a weaker weapon, killing them. "There's only 199 Mooks. I don't need to use the BFG."
Played For Drama: The BFG was given to The Hero as a one-time trump card by his late father, and he can't use it out of Angst.