I've found that on my AMD64 box, when I press the left and right buttons
simultaneously, I get a middle-button event.  This mouse has a proper
middle button, and I have never asked XFree86 explicitly to emulate the
third button this way.  I cannot see any sign of the feature being enabled
in /etc/XF86Config, either.

I have not seen this on other systems, but have not looked very hard for
it, either.  It is rather annoying and I'd like to turn it off.

I thought that I'd ask here, first, to see if anyone else can confirm
this behavior or maybe suggest a way to fix it.  (For all I know, it
could be a "feature" of the Logitech USB optical mouse that I'm using...but
posting here seems like a place to start.)

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/