Comment 2 for bug 1144631

If you use dconf-editor, go to org/gnome/desktop/wm/preferences, you can go to button-layout, and change the text there to "close,minimize,maximize:" being sure that the semi-colon is at the end of that. Hit enter and buttons should be as they were. Odd that this just happened, I see no reason why it would, given that maximised apps still show buttons in the top left of the panel.