It’s back: the dreaded curse of “Tristan.” At the Metropolitan Opera, Gary Lehman, above left, sang in place of an ill Peter Seiffert, above right, in Tuesday night’s performance of “Tristan und Isolde.” Last season, Mr. Lehman was one of four tenors to sing Tristan in a six-performance run because of various illnesses and mishaps. Mr. Seiffert drew attention during the current run for wearing an earpiece to receive electronic cues instead of relying on the usual stage prompter. “Tristan und Isolde” continues at the Metropolitan through Dec. 20.