This happens in a race condition where a player requests an update right before signing off, before the reply is sent back. I think it's unavoidable in the general case (but should probably investigate to see if Blizzard offers an atomic checkIfOnlineAndMessage API), but could be improved by:

• Checking if the player is online right before replying
• Fixing #12 so there isn't quite so many replies going on -- sometimes the "no player found" error pops up a dozen times right in a row

Turns out WoW has no API for checking if a player is online by name; the best you can do is a WHO request, which is asynchronous and infeasible. #12 is fixed now, so that will have to do