I'm sure we've all been privy to the wonderful world of mis-tells/types. Unfortunately for us typists there is no real way to avoid this as of now.
How would you feel about adding PM boxes to the UI? something that can be minimized and keep the conversation with that one person private and separate from the rest of your chats. Seeing as how /tell is used for more than personal conversations I hope that it doesn't automatically open a new box, but instead if you click on their name it will give you the option.