I bet it's the back button.
I usually get it by pushing the mouse left until it hits the screen border
Too bad this actually brings up the left bar on Ubuntu and it covers the button itself.
This actually requires to move the mouse again to clear the bar and then retry to center the button again.
Bad design decision.
Time to give Gnome 3 a try.
I've found that I'm not the only one that stresses the importance of the back button: