In the early days of digital I had a Pentax compact & a Nikon DSLR that both took Compactflash cards. Whilst on holiday I needed a fresh card for my Nikon so I used one that had been formatted in the Pentax and just deleted files to create space. When I looked at the files later all the early ones were fine but as it got fuller the files were corrupted. A local camera shop helped me rescue most of them and explained that the corruption arose when the Nikon tried to write over areas that had previously been used for Pentax files.
I now do two things to avoid a repeat.
1. I always format fresh cards in the camera in which it is installed.
2. I keep separate sets of cards (Now all SD) for each camera type - now Canon & Nikon.
I have no idea if this is necessary but I have not had a corruption since that first one.
