An AppleScript used to select a project in Aperture may generate the error message “Aperture got an error: Can’t get project ‘yourprojectname’ when run on Mac OS X 10.5 Leopard. The same script runs without an error message on Mac OS X 10.4 Tiger.
This may happen if you do not specify the library to be used in your script. This has always been the best practice, but it was not mandatory under OS X 10.4.x.