Copyright issues and public performances:
"Due to the copyright issue, filmmakers rarely show complete singalongs of "Happy Birthday" in films, either substituting the public-domain "For He's a Jolly Good Fellow" or avoiding the song entirely.
One of the popular audience lines in The Rocky Horror Picture Show alludes to this. After Dr. Frank N. Furter has captured Brad, Janet and Dr. Scott, he hosts a dinner for them. At the beginning of an apparent birthday celebration, the audience calls out "Start to sing 'Happy Birthday' but don't finish it", and indeed, Dr. Furter cuts it short midway through the song."
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