* Abrasions: your turtle might be rubbing itself agains some
rocks in the cage. Remove the rocks and it should heal.
* Poor shedding. If your turtle does not shed scutes well, water
collects under partially shed scutes; a paradise for bacteria.
* Bites by another turtle.
* Husbandry issues, leading to shell rot.
* Somebody actually drilled a hole into the shell
* dog bite