Comment 3 for bug 479489

In some cases the zoom "feature" is not reversible: if you have a non-scroll-wheel mouse, or a laptop with three mouse buttons, you can zoom in but you can't zoom out without a) realising what you've just done, and 2) finding and attaching a scroll-wheel mouse. For example, if you're using a thinkpad which has 3 mouse buttons beneath the space bar, sans external mouse, and press Super+MiddleClick, you're screwed.

In case anyone else finds this: the only way I found of "fixing this" was to install Compiz Config Settings Manager, and disabling the Zoom plugin.