I think this happened around a week ago. When I open the Unity Dock, the dock covers the whole screen instead of just the upper left corner of the screen. I've made another user account to test this, and it does what its supposed to on the other account. I'm not really sure what happened. The dock still works fine, but it's kind of annoying and I think it is slower when it covers the whole screen. Anyway, if anyone has any suggestions as to how to fix this, please let me know.
btw, I use Ubuntu 12.10.