Circle Mesh - Can't Adjust Vertex Count I recently tried to create a circle mesh, which works fine. However I cannot change any of the Add Circle settings. It is all grayed out, so I cannot change how many vertices there are in the circle with the sliders.

Is anybody else having this issue? Is there a way to fix it?
Oh, I found the answer! Go to File > User Preferences > Editing > Undo, and enable Global Undo. Problem solved! :)