Well, shell rot can is characterized by white, dull patches on the shell. In its early stages it is easy to care for but if you let it progress to long it will eventually wear away at the shell until bone is exposed. Extreme cases wear away the shell entirely. It is fairly common ... it is caused by an excess of bacteria and/or fungus growing in the water.
Mild shell rot is easily treated. First, you want to let your turtle's shell dry completely -- put him in a tupperware container or something similar for treatment. Next, you want to apply some betadine solution (iodine antiseptic) directly to the affected areas with a q-tip -- if you don't have any solution on hand you can get a bottle in the first aid section of your drug store. It works as a disinfectant against whatever is already growing there. Next, apply an antibacterial ointment. Your should keep your turtle's shell dry for at least the next few hours to let the treatment work. Repeat the treatment in a few days. If you don't notice any changes than switch to an antifungal for the next treatment. Sometimes it can take awhile depending on the severity of the shell rot ... turtles aren't fast healers. But, if your turtle does have or ever get shell rot don't put off treatment because you don't want to wait too long or your turtle will need a vet's treatment instead.