In photography, accidents happen. A photograph could be out of focus or poorly exposed, or the composition could be off. Maybe the photographer just snapped a frame without noticing.
One example I found recently is on film scans. These may or may not be considered photography at all, but I like them. I sent in some old film lying around in my studio to the lab. I had no idea what was on these rolls of film, some more than a decade old, and I told the lab to scan whatever they found.