This work is licenced under a Creative Commons Licence.
If you have multiple monitors in Ubuntu and are using the new Unity graphical interface, you may find that the "vertical bar" is appearing upon the left hand side of a screen not the left-most. This is weird, and means you have a vertical bar in the middle of two monitors. Even worse, as it auto-hides by default, you have to hit a very small area (possibly as little as a single pixel) between two monitors to activate it.
It seems that setting the left-most screen to be the "default" in the monitors app should fix this, but for me it did not. I had to edit ~/.config/monitors.xml and change the value in the correct monitor from "yes" to "no" for the "default" attribute. It's raised elsewhere as a bug here: https://bugs.launchpad.net/unity/+bug/742544