Occasionally, when I'm fighting a mob, my autoattack is turned off continually. If I use an ability that turns on autoattack, I get an "autoattack on" message, then the ability message, followed immediately by "autoattack off." The same is true if I use the autoattack hotkey by itself. This persists throughout the combat, but it returns to normal for the next combat. I would say this happens every dozen to 15 combats.