It was a soggy 1966 matzoh ball called Chu Chem, which actually never made it to Broadway–it closed in “Killadelphia.”
As a former publicist tells Peter Filichia in the latter’s new book about hit and flop musicals:
“[Leading man] Menasha Skulnik had to stop the show to remind the orchestra that the tune they were vamping on had been cut.
“He also told the audience, ‘It was a terrible song.’
“He began the second act by delivering the news that Molly Picon, his costar, wouldn’t be able to continue with the show.
“He did keep from us, though, that she had quit for good during intermission.
“So he introduced her understudy, Henrietta Jacobson, who was about twice Picon’s size.
“As a result, she couldn’t get into Picon’s costumes, but simply pinned them on to what she’d happened to wear to the theater that day.”
By the way, a 1989 revival of Chu Chem did way better–it actually made it to Broadway! (For two months.)
And if you’re wondering what the top Broadway embarrassment of all time was, you haven’t been reading this blog carefully enough.