Scope: Campaign with lots of political moves in the background. There was a dispute over who should rule kingdom after the king died, as his son was too young to rule. We both did some killing of those who seized power, and protected the young boy. This particular part of the story takes place in the middle east during the crusades, shielding the boy from harm by taking him to the last place anyone would expect us to go.
System: Homebrew rulesystem, that used, of all things the d12 as the core dice!
Style: Low magic fantasy using real world history as backdrop. Magic existed and was not entirely uncommon, but it was generally less powerful than melee and you were more frail (in an already very lethal system).
Demographic: Two late teens, one young teen. It was a skill based not class based system, but the characters would be best summed up as Necromancer(me), Ninja, and Fighter/Rogue.
Memory: My character had a middle eastern heritage, so he had been playing along with the "Muslim" faction, when he noticed that they were headed to the home where the boy and the rest of the party was staying. I had to convince them that I would handle this house alone. I ran in and we all stared at each other trying to figure out what to do. The inspiration came when I realized an artifact I had could be used for something completely unintended*: to use his feign death power that was usually constrained to himself on those in the house. But it had limited uses and one person would have to be left out. Then the Ninja offered himself to be killed so that the others would not be found out. So my character had to kill someone who had become a friend, and then charge outside having done the deed in a fashion believable enough for it not to make the sacrifice meaningless. (No diplomacy checks here, you act it, or you fail)
Analysis:
---This story: Having to think quick and find a solution to what seemed an impossible situation, and having it be a solution that no one liked but what had to be done.
---The whole campaign: Playing something as shunnable as a necromancer is a setting/system where it actually meant I was frail enough to die if found out. In say D&D or WoD, I could just say F'it, and flaunt my powers and just kill anyone who tried to stop me. I was still powerful in this system, but the entire world was much more lethal than any other system I have ever played.
Components: This one is really hard, because this example really had these all singing in chorus.
the actual characters - For making that situation necessary. Each character had reasons why protecting the boy was vitally important to them.
the mechanics - For making the consequences very real.
the gamemaster - For recognizing this situation had presented itself and pushing it forward.
the other players - For selflessly sacrificing himself for the greater good.
the story - There wasn't really any railroad here, just events within and beyond our control that we had to react to. Additional kudos to the DM, who took it in stride when we took it in stride when we decided to run far away to protect the boy.
the setting - Being history through a lens was pretty cool, as you always knew and simultaneously didn't know what to expect from an organization or place. (Tangent! Very excited because another person is planning to run a campaign with a similar semi-historical basis.)
* - The item allowed me to use necromantic powers that could normally only target myself. Was originally intended with offensive abilities in mind, like using my life stealing ability to heal an ally instead of myself.