are you using the same Aperture library on both the machines, or is your issue independent of the Aperture library you are using? Does it happen also when viewing a new library with some trst pictures?
Have you deleted your preferences yet? Doing so sometimes resolved odd UI issues like this.
You can do so by quitting Aperture, then, from the Finder, hold down the "Option" key and go to the Go menu. Then select Library. From the window that pops up, go to the "Preferences" folder, and move the "com.apple.aperture.plist" file to the trash.
Just tried deleting the prefs file but unfortunately it still does the same thing now and then.
Haven't tried creating a test library but will try to tomorrow if I get chance.
Regards, thanks and good night,
Dave, have you logged off and on after quitting Aperture and before removing the prefs? If not, then try again, but this time log off and on. Otherwise you cannot be sure, if the Aperture process really has been terminated by Mac OS Mt. Lion before you remove the prefs and Aperture simply may be using a cached version.