You do a rather mundane favor for a stranger (helping him find the library, paying for the coffee he ordered but couldn’t afford, something like that) and he is grateful to you beyond all expectations. Such kindness must be rewarded!
The stranger explains to you that he is a visitor from another realm, a land of great magic, where he is a powerful wizard. Favors carry great weight in the land where he comes from, and so he must present you with a gift.
He gives you a gun loaded with a single bullet. This gun, which can be fired only once, will hit anything or anyone anywhere in the present or past. You can shoot Hitler lying in his crib, or you can shoot Kitty Genovese’s attacker just as he lunges at her. You can shoot Constantine while he dreams about the cross, or you can shoot the abusive creep your sister married.
You could, if you wanted, shoot a hole in one of the Apollo landing modules, or into the engine of the Enola Gay. Maybe you could even shoot out the lock holding shut the doors of the Triangle Shirtwaist Company.
You mention, politely, that there is a long storytelling tradition in our world in which magical gifts such as this one always operate in such a way to harm the person using it in an unexpected way. The sorcerer laughs at this in a startled way, as though you’ve said something scandalous, then assures you that such a person would be an infamous criminal back home, and then assures you that there is nothing sneaky or underhanded about this gift. It will perform as described.
Do you fire the gun, and what do you shoot?