People that drive too slow or are just in my way when I am in a hurry.
Stupidity.
People not doing their job, or only half-[butt]ed doing it. This one is extremely frustrating due to what I do on my job. Why bother to set stock level maximums when jack-[butt] will shove an entire pallet in the location instead of the four boxes it calls for? God forbid he actually follow what has been set down, even if it means he might have to pick up four boxes and put them in the rack...