gorilla arm

gorilla arm n.  The side-effect that destroyed touch-screens
   as a mainstream input technology despite a promising start in the
   early 1980s.  It seems the designers of all those {spiffy}
   touch-menu systems failed to notice that humans aren't designed to
   hold their arms in front of their faces making small motions.
   After more than a very few selections, the arm begins to feel sore,
   cramped, and oversized -- the operator looks like a gorilla while
   using the touch screen and feels like one afterwards.  This is now
   considered a classic cautionary tale to human-factors designers;
   "Remember the gorilla arm!" is shorthand for "How is this going
   to fly in *real* use?".