Ever gone back to edit something, and have the letter right after your edit disappear when you type another letter?
(ie, Somthing becomes Somehing and then Someting?)
You may have (accidentally) hit the INSERT key, especially as it is right next to the DELETE key on most keyboards. This will overwrite the letters with whatever you type (and it a holdover from the typewriter days). Try hitting the INSERT key again and seeing if the behavior stops.
Some programs used to have INS or such appear the in the status bar, but most nowadays have no indication the button has been pushed. It also doesn’t light anything on the keyboard like the CAPITAL, NUM and SCROLL LOCK keys.
Of course, hitting that, if it wasn’t the cause of the strange behavior, will only make things worse. Like a computer can get any worse. (grin)