Papa told her about a Lohengrin performance. It was just before his first entrance. He was ready to step into the boat, which, drawn by a swan, was to take him on-stage. Somehow the stagehand on the other side got his signals mixed, started pulling, and the swan left without Papa. He quietly turned around and said: "What time's the next swan?"
That story has since become a classic in operatic lore.

Walter Slezak, in What Time's the Next Swan? (1962), p. 210