At the very end of my installation process for upgrading my laptop to Mountain Lion I suddenly found that my mouse cursor on my computer screen had an annoying black box associated with it.  My son helped me determine that the installation defaulted with a system preference set incorrectly.  If you find you have a black box following your mouse cursor all over the computer screen just go to System Preferences (top left corner of your computer screen and select the Apple icon to view this option).  Then go to Accessibility and then Zoom.  Turn off the toggle selection box “Use keyboard shortcut to zoom”.  The box will then disappear.