system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did it take 100 years to discover the happy birthday copyright was invalid? And do they have to repay the people they collected royalties from?
gpt
Simply put: the US Congress passed a law in the 1920's that extended copyright for works in publication. 

Fast forward to the 1980's / 1990's, and Warner Music has bought the papertrail that demonstrates copyright to Happy Birthday, including a publication of the lyrics with a piano arrangement of a melody, and use that to show that the copyright would be extended by that law to the present day.

Fast forward to the 2000's, and an archivist / researcher uncovered documents showing that Happy Birthday was in print and copyrighted *before* the documents held by Warner Music, and that the publication date of those records demonstrate that the copyright on the lyrics expired *before* the law passed by the US Congress in the 1920's would take effect.

All that's left to Warner Music is a few particular melodic arrangements *for piano* that were published as set to those lyrics; vocal arrangements etcetera aren't covered, the lyrics aren't covered.

Whether the people who paid royalties get them back depends on what they do and what courts decide. To get that back, they probably would have to prove that Warner Music was aware of legitimate documentation that demonstrated the copyright expired, while they were billing for royalties.