>I have not heard of a camera overwriting images on a full card >before.
I, unfortunately, have, and it was caused by a knock-off memory card, labeled as Sandisk. They use smaller memory chips and write the firmware so that when you reach the end of the card, it starts overwriting from the start again. Usually these cards are easy to detect though, as their prices are far too good to be true.
I would suggest looking at the custom settings as suggested, that seems the more likely culprit by far.