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

afterward. 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?".