Pasting text with a middle mouse click produces double what is selected. For example, selecting "localhost" in a xterm, then pasting with a middle mouse click (I have a logitech wheel mouse) produces "localhostlocalhost".
Torrey T. Lyons
Actually both the 2nd and 3rd mouse button are doubled. The other one just isn't as obvious. This only happens with the Cocoa version of Xmaster.