A new feature was added recently that adds an invisible border around the windows to grab for resizing (although i thought the new resize grip solved any big issues).
This annoys me because it destroys the usefulness of the grid plugin of compiz.. i'm not sure if the border is part of compiz or gnome, but i'd like to know how to disable it.
i couldn't find any options in ccsm or the window settings in gnome.
See the screenshot to see how much waste is caused. These windows should match up instead of having blank space surrounding all of them.