You probably accidentally hit the Insert key on the keyboard. That is a toggle switch to let you either insert extra letters or type over the existing letters.
when I was typing an email, i tried to go back and fix an error and the curser which worked perfectly a second ago, seemed to go backwards and was erasing a different part of the word that I needed. The volume also changed. I powered down and everything went back to normal but I am worried. If this is something that can be fixed with a program please let me know. It doesnt happen all the time but I am concerned.