Look at this radiograph. There is a thick black shadow at the bottom of the x-ray. What do you think has happened?
Apparently, while the radiographer was about to pull out an unexposed film from its box in the darkroom, a friend came by and accidentally open the dark room door. The part of the film that was exposed is now fogged. Being a smart radiographer, he continued to use the film but by making sure the fogged part is placed inferiorly.
Compare this to the well-collimated film below. The collimated portion is unexposed to x-rays and thus (after processing) should be clear.