I'm not sure if this is way off topic but I think that happens with me too.

It happens in Notepad, when I'm typing some coursework up, and made a typo I go back using {LEFT}. When I try to correct the typo it overwrites the text after the typo.

In mIRC also, but as LostServ said if you click "Ok" in the remote editor and then re-open remotes it seems to work again.