I had this happen once with some sixty-year-old infrared film! The first couple of sheets in the box turned out already to have latent images from the Portland/Vancouver area. Fortunately I'd exposed them through an IR filter and the film's IR sensitivity was long gone, so I didn't blow away the previous owner's shots.

(The images came out mottled in the manner of film that's sat around undeveloped for a long time, so I assume they were latent, rather than being developed negatives that someone had put back in the box. But I can't be 100% sure.)

-NT