I'm on Unity, trying to configure my collection of monitors. Until this afternoon, I could click on "systems settings", select displays, and click-drag to rearrange the monitors on the screen. This resulted in reasonable behaviour - the mouse could move across boundaries from one to another, and windows could be dragged between them.
This afternoon I acquired a third monitor. In particular, I'm now on a laptop, with 2 large monitors attached to its docking station - both rotated 90 degrees, and the laptop screen open. I was initially able to drag monitors around, but various weird things happened, including an all-screens-dark lock up that forced me to reboot.
Now the only way I can get decent behaviour is to turn off the laptop screen. Even then, the two external monitors overlap in the "displays" screen, and cannot be moved.
If I disconnect one of the monitors from the docking station physically, by removing the cable, behaviour changes, but things are still flaky. (If I recall correctly, I was able to move the laptop's screen vertically with respect to the external monitor, but not move it from one side to the other. And lots of windows completely disappeared - dragging them around within the "workspace switcher" helped me to regain access to most of them.)
I'm on 12.04 LTS, desktop installation. (Possibly 12.04.01.) Update manager thinks I have everything up to date - but don't trust that too far, since it's lied to me on other Ubuntu desktop systems.
[Update: One more detail - when I attempt to drag the monitors, most of the time nothing happens. Sometimes they swap places, i.e. they are still overlapping, but the one that's physically on the left shows the right side of whatever's on my desktop, and I have to move the mouse off the left edge of the leftmost screen to get it to appear on the right one.]