Lack of planning is my bugaboo.  I can't count the number of times I've had someone jumping all over me to book a trip NOW! NOW! NOW! but they haven't finalized all their plans yet.    So I end up making endless changes to flights and hotels and car rentals while they finalize their appointments. 

And it's usually the very last minute when everything is settled, so then I hear whining about how the hotel they wanted is booked solid and how the car rental company only had a sub-compact left, and how they can't get an aisle seat on the plane.