Say you typed the word Middle as "midle" and you go back and try to add another "d". Instead of adding another "d" the computer just replaces the "d" with another without adding a 2nd one. Often I find myself adding a sentance in am essay ect. and I am looking at the paper I am copying from and without knowing, what I am typing is erasing what was previously there and replacing it. This has happened to me on my laptop and my desktop and multiple other systems. It doesn't happen very often but it one of those annoying little bugs. The only way I have found to fix it is to reboot the system. What is this and why does it happen?