The New York Times said this was the second consecutive time in the six-performance revival of Wagner's opera that something interrupted a performance.
Deborah Voigt, who played Isolde last Friday, left the stage during Act II because she wasn't feeling well and was replaced by her understudy Janice Baird, who made her Met debut.
Lehman also made his Met debut Friday, replacing John Mac Master, who was the original replacement for Ben Heppner, who had canceled his first four performances because of a viral infection.
Lehman reportedly replaced Mac Master because Mac Master was struggling with the role due to allergies.
Lehman was not seriously injured when the set malfunctioned, sending him sliding into the prompter's box. After a brief break, he finished the performance.
"It seems this production's run has been somewhat star-crossed," Peter Gelb, the Met's general manager, told the Times. "There are risks in any production, but certainly what happened was a fluke, and when you consider that the Met is a repertory theater, with so many different performances, it's remarkable how few mishaps occur."