There have been many operatic disasters over the years. One of the most infamous is the "bouncing Tosca" (or "Trampoline Tosca"), but this may be an urban legend.
The bare bones of the story are simple. At the end of Puccini's
Tosca, the heroine cries out "Scarpia, davanti a Dio" ("O Scarpia, [we meet] before God!") as she faces her death at the hands of the sadistic police chief Scarpia. After this, she hurls herself off the battlements of a castle, and dies. (Actually, the actress lands on a mattress about four feet below. Sorry to spoil the illusion).
But in this case, the actress landed not on a mattress but - perish the thought! - on a
trampoline...
I've heard this story many times, and every time it's a different actress (Ljuba Welitsch, or Dame Eve Turner, or Anna Tomowa-Sintow), different opera houses, or different circumstances (e.g. the stagehands hated the diva, and this was their revenge; a stagehand was inexperienced, and tightened the trampoline to make it extra bouncy, etc.)
Perhaps every diva wishes it was her?

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One well-known (and undoubtedly true) story is as follows:
Leo Slezak was a dramatic Austrian tenor, who made his debut at Covent Garden in 1900 and performed until his untimely death in 1946.
In Wagner's
Lohengrin, the onstage actor has to get into a skiff drawn by a swan, which then disappears offstage. However, during one performance, a stagehand pulled the swan off the stage too early, before Leo could hop aboard.
Seeing his feathered transportation disappear into the wings, Slezak ad-libbed to the audience, in the manner of an annoyed traveler addressing the stationmaster: "Wann fährt der nächste Schwan?" ("When does the next swan leave?")