I remember reading somewhere about someone creating a stone door closing when they were packing up for the day and scraped a box along the ground that was covered in salt and I'm sure there's many other examples. So what's the best thing you've made by mistake?
stepping backwards away from a lecturn and laughing turned in to this amazing phazey sci-fi sound that ended up being a car-start for a futuristic show....
I have a folder full of 'leftovers' that holds all the little gems that might otherwise be indiscriminately culled from a recording session. Things that just sound cool and are hard to categorize.