As the world's slowest typist, I used to find that sort of thing happening to me a lot - but there is a simple solution.
What I do nowadays, whether it be a message to the bank, my ISP, whatever - anything that's going in a box online - is knock it out offline as a plain text file first (i.e., if using Windows, use Notepad, not something with formatting like Wordpad or whatever) then copy and paste it into the box.