What was the most unusual and innovative technique of capturing or generating sounds in your entire experience with sound design ?
What you have you been forced to do, to achieve the goal?
Neither of these are too extreme, but what the heck.
I once placed my windscreen/zeppelin on guy wires while striking them, in order to both capture the in-air sound as well as the low-frequency mechanical vibrations, not unlike a contact microphone. Worked pretty well for that single purpose.
I also tried using boundary layer mics for recording urban stereo ambiences. While not wholly successful, it was a great learning experience.
(This is in addition to the standard silly things we all do: putting cheap mics in condoms for underwater recording [EDIT: unlubricated!!], wearing minature mics in public, strapping mics to moving objects in dicey places, dangling mics in front of things that might eat them, et multiple cetera.)
I once caused a large traffic jam in Belfast. I needed lots of different car horn sounds to recreate the sound of Mumbai. My wife and I stood next to a dual-carriageway, I had an MD recorder and my wife was carrying a sign asking people to toot their horn.
Got loads of great toots. Even people from Mumbai have said that the film perfectly captured the atmosphere of the city.
+1 for using condoms in microphones, works really well.
I don't know about most unusual, but I like to record things from the far end of long pieces of PVC pipe, to use the resonance of the pipe as a filter. Another good trick is to drag the mic through the pipe from one end to the other while recording, so the character of the filtering changes over time. Put it on a sock or something to reduce handling noise.
Mine is mildly unusual but I'm proud of it :) I can make rain on a window with only my fingers, and a glass pane (like a window...). I found that fingers tapping on the glass sound just like water drops if you find the right velocity. So for a light rain, just record your finger tapping for 2 minutes, then expand, split into four 30 seconds tracks and here's your light gently hitting the window.
Heavy rain? Lay the glass pane flat, spread some water to give a bit more impact to your fingers and get on with the tapping. You can call in some friends to make the tapping a social activity and to vary finger textures :D