Once when installing chimney pipe in a frame chimney chase of a cottage in the cedar forest in Michigan's UP, I had to spend about an hour up in the very top of said chase while others worked either below me or outside on the roof. After a short time, I noticed that there was a beautiful projected inverted image on one of the walls of this small area. Apparently there was a small nail hole near my head that let me experience the space as my private camera obscura. Serendipitous events are the best!