From Photoshop News.com
Apple has posted a TechNote for users of OS X 10.5 Leopard with Time Machine and Aperture. By extension, it also applies to Adobe Photoshop Lightroom users as well.
Issue or symptom
Running Time Machine backup or restore operations while Aperture is running may lead to inconsistencies in the Aperture database.
Products affected
Aperture
Mac OS X 10.5 Leopard
Solution