Weird black bar in dock

[marddin]

The above black bar is showing on my dock. The only way I can remove it is by restarting although after a few minutes it pops up again. Anybody know what this is?

Thanks!

 

[Nermal]

I've had it too, and there's another thread floating around somewhere. The quickest fix is "killall Dock" in Terminal... and so far it hasn't come back for me.