I suppose this isn't exactly an accident, although it was both unplanned and unfortunate.
I bought my first 4X5 camera (an old Busch Pressman) right around Christmas of 1997. It took me a a few days to figure out how to load up film holders, focus on a ground glass, etc. So after about a week, I was finally ready to shoot. I exposed one frame on my sun porch, and then drove out west of Albuquqerque looking for anything at all with interesting texture. I found a decaying adobe barn, set up the camera, and took one shot (attached); only the second 4X5 sheet I'd ever shot at that point. I thought that was a fairly unintersting composition, so I left the camera where it was and started walking around looking for something better. When I got back, I found a horse licking my precious 4X5 camera. Fortunately, I had the presence of mind to shoot a quick 35mm snap with an old Bolsey B2 I was carrying (attached).
The Pressman was slimy and dirty, but cleaned up ok eventually.