This message is expected if the file is damaged or if the project is from a later version of Final Cut Pro than the one you're using. (For example, Final Cut Pro 4 is not expected to open a Final Cut Pro 6 project.)
If Final Cut Pro 6.0.2 is running on Mac OS X 10.5 Leopard, this message may occur in situations where it is not expected.
To resolve this issue, update to Final Cut Pro 6.0.3. The Final Cut Pro 6.0.3 update is included in Pro Applications Update 2008-01. Choose Software from the Apple menu to update your software, or click here.
This behavior can occur in cases where the expected behavior is that you should be presented with the Offline Files notification. When this issue occurs, instead of being able to reconnect (or disregard) offline files, you are unable to proceed with opening the project.