If you take a song that you created in GarageBand version 1 or 2, open it in GarageBand 3, and save it, then it will no longer open in GarageBand 1 or 2.
This happens because GarageBand 3 has a new song format that makes its projects incompatible with earlier versions. To maintain backward compatibility, create copies of older songs before opening them in GarageBand 3.
Note that there's no warning messages about this compatibility issue.