Sometimes what you set out to record ends up being much less impressive than that random "sound of opportunity" you happened to stumble across on the journey. I believe Ben Burtt was looking for starship source from rocket tests at White Sands when he discovered that the broken aircon unit in his room sounded more like what he wanted than the rockets he'd come to capture.
One of my own favorites would have to be from a trip to Texas a few years ago. We were there to record a history-drenched C-47 aircraft for a World War II game (I was lucky enough to record the interiors in its second-to-last flight before it headed to the D-Day museum in New Orleans to go on display. It was amazing, if a bit nerve-wracking on failed takeoff attempt #1). We did get great stuff from the plane, but also stumbled across a huge weathervane (we think) at the airfield that looked like a giant metal arrowhead. We miked it from within/underneath and attached a contact mic for good measure, then rotated it slowly for some incredible low metal stress groans.
What are your favorite happy accidents/surprise discoveries from recording expeditions?