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