What is this black bar at the bottom of my Dock? I have this strange bar at the bottom of my Dock:



What could it possibly be and how could I remove it? It is non-blocking but distracts me when using the Dock.
Occasionally the progress bar that appears under the downloads folder does not go away as it should when the download completes. You should be able to remove it by simply restarting the computer or running 

killall Dock

at the command line (Terminal). This should reset just the dock process without having to restart the whole machine.