At a performance of Strauss's ARIADNE AUF NAXOS at The New York City Opera in 1974 - which I attended - the tenor was clearly losing steam as the strenuous final phrases of the opera loomed before him. Suddenly, the evening's Ariadne, Johanna Meier (photo above), began to sing along with the tenor. bringing the opera to a splendid close.
Johanna Meier rescues the finale of Ariadne auf Naxos - NYCO 1974
An associated story here.