It happens because it is one of those bugs that you wouldn't necessarily see unless you use the software in a particular way. NX 1.3 works fine on Leopard for the most part, but issues can arise in the following situations:
Exporting TIFFs of up-ressed files Deleting crop steps from the Edit List.
If you see black squares appear on your image, DO NOT SAVE YOUR FILE. Revert to an earlier version, or close and reopen it. Saving a file with black squares will lead to file corruption.
Jason P. Odell Colorado Nikonian Author, The Photographer's Guide to Capture NX Now supports Capture NX 1.3