On the eve of a judge's ruling, attorneys find key evidence tucked away in the files of the opposing side. They rush to court with word of their bombshell discovery.

The filmmakers working on a documentary about the world's most popular song, "Happy Birthday to You," and currently suing Warner/Chappell for the right to use the song in the documentary without any license fee, filed court papers on Monday touting newly uncovered evidence that "proves conclusively that there is no copyright to the Happy Birthday lyrics."

The "proverbial smoking gun," as the plaintiffs put it to a California judge, is a book of children's songs that comes straight out of Warner/Chappell's digital library.