If you @ mention a username, and the beginning of that full username happens to match a shorter username, it will complete to the shorter username instead of the longer one.
Basically a duplicate of http://www.elkarte.net/community/index.php?topic=3873.0 (just because I'm pretty sure the "-" of the other topic is not the cause of the problem, but the fact the name is shorter is the real issue)
Right, sorry. Thought it was a different issue with the same cause.
Nothing to be sorry about. :)
Better open two reports than none. ;)