I encountered a head-scratcher today. As I helped a client with a layout, we were trying to locate an elusive image on their file server. We were using Adobe Bridge (of course!) to browse the files visually, but we noticed that some of the images with a .JPG extension did not display thumbnail previews.
Photoshop gave me an error when I tried to any one of these images. But the same images could be placed in an InDesign layout. That seemed strange, so I selected a placed image and looked at the Info Panel (Window > Info) in InDesign. Sure enough, the Info panel said that the image Type was PNG, not JPEG. Someone had tried to change the image from PNG format to JPEG format by simply changing the filename extension. This, of course doesn't work. To change image file formats you need to use a conversion utility or open the image in Photoshop and choose File > Save As.
Without this bit of information in the Info Panel, I probably would have wrongly concluded that the files were damaged. Changing the filename extension back to PNG allowed the image to be opened in Photoshop.