Back in 1993, I was at an early matinee preview of the first part of Angels in America, Millennium Approaches, at the Walter Kerr, well before it became a well-deserved phenomenon. I was in mid mid-20s, but most of the audience was comprised of older folks from theater clubs, presumably bused in from the 'burbs. Needless to say, there were a lot of walkouts!
Even though I was sitting way in the back, I thought the performance was spectacular, and don't remember any technical issues until the very end, but boy oh boy it was a doozy. Picture Prior Walter cowering in bed, lights flickering, set rumbling, debris ceiling debris falling, and descending down from on high -- no one. Nothing. Until finally, after what seemed like forever but was probably only 10 seconds or so, a panicked voice comes shouting over a walkie-talkie, clearly audible, at least to those of us sitting near the technical booth/sound board at the back: "Cue the angel!" After which the Angel does descend to declaim one of the most famous ending lines in 20th century theater: "Greetings Prophet; The Great Work begins: The Messenger has arrived!"
It certainly didn't ruin the play for me, and I was back there in the fall to see both parts together when "Perestroika" opened. But it's hard to imagine a bigger mishap at a worse time. Then again, that's what previews are for!