What's the most formally ambitious scene you've ever run?
I'm not talking about challenging in terms of, "this scene is critical to my 10-year-long campaign, and I really need to hit these notes perfectly" or "there are 25 cave-trolls fighting 6 brass dragons fighting 3 PC's". (These are certainly challenging to run, but in a different way.) I'm thinking more like, "Art Film" style GM'ing-- scenes were time isn't operating in a linear fashion, scenes that operate entirely in dream-logic, scenes where there are weird communication rules between the (real life) participants, etc. etc. What scene have you run that made you say, "Damn, I can't believe I pulled that off?" (or, "Argh, if only I hadn't been so madly ambitious"?)
Mine occurred this afternoon, when I improvised a scene set entirely in another player's hallucinations, which served (a) to challenge the player's most cherished trait, (b) to provide an info-dump on the villains' true goals (which were entirely improvised but 100% logically coherent based on the material we'd had so far), all while (c) obeying David Lynch-style dream logic. There was a little bit of flailing around, and we had a slightly difficult time figuring out when to trigger the conflict rules, but I was really pleased at the scene overall.