Number one on my personal list:
Prioritizing the wrong activities –
Busy work. It’s quite literally work that keeps you busy; it saps your time, but gets you no closer to your end goal. Ineffective people tend not to recognize busy work, and therefore, they prioritize tasks that will not move them any closer to their goals. Effective people recognize busy work for what it is and waste little to no time trying to appear busy when they know there are more important tasks to be completed.
http://www.quora.com/Chris-Wake/Hidden-habits-of-ineffective-people