Werther from Massenet, third act, at the San Diego Opera. After the intermezzo swells to a dramatic climax, the music suddenly stops. The stage is completely dark, and at this moment, a small spotlight shines on the tenor, Werther. He raises a pistol to his head, the light goes out, and the darkness is shattering by a gunshot. The music continues, and when the lights go back on, Werther is lying on the ground, dying (it takes the entire act for him to die, and he still manages another seven high C's!).
But on opening night, after Werther pointed the gun to his head and the light went out, all we heard was "click.. click, click... ...click, click, click... ...oh, bloody hell!!"

The following performance of Tosca took place in Madrid. It really happened -- I saw it with my own eyes!
Tosca was about one head taller than Cavaradossi, so when they embraced at the end of the third Act, she ended up pressing his head into her breast, as a mother would embrace a child. Cavaradossi was just able to turn his head in the last second to avoid suffocation. The audience loved it.
Tosca then heard the firing squad coming and hid behind some canons, to watch her lover "pretend" to be shot. After Cavaradossi was shot and the firing squad left, she ran up to him and discovered that he was dead. She screamed "Ah!!" so loudly that he jerked his head to one side and rolled over. As the audience roared with glee, she ran up the stairs, knocking over a pyramid of stacked Styrofoam "canon balls", which, glued together, fell end over end down the stairs.
As Tosca reached the top of the stairs, she ran to center stage, sang "O Scarpia, avanti a Dio!" and turned to hurl herself to her death. But she hesitated, gingerly pulled up the hem of her dress, and stepped carefully down from the platform instead. When she stood to leave the stage, the explosion of laughter from the audience signaled to her that she was now visible from the waist up. She made a expression of "oops", crouched down behind the platform and, as the curtain fell, we all saw a 10 meter high shadow of a woman on her hands and knees crawling offstage.