I read about a production of Macbeth where a weapon broke during every single performance in the 4-week run, but one instance was spectacular.
"During this particular performance, we had gotten to the Macbeth vs MacDuff fight without a weapon breakage thus far, and everyone was holding their breath to see if we'd get through the performance weapons intact. On the third blow of the fight, Macbeth's sword shattered. The sword blade flew backwards and embedded itself in the proscenium, the pommel dropped to the stage, and the quillons flew upstage, striking the cyclotron, leaving the actor standing there in front of God, the audience, and MacDuff holding the grip of the sword and with a full four-minute swordfight to perform. The actors improvised a brilliant grappling fight, ending with MacDuff standing over the prostate and pummeled Macbeth with his sword raised to decapitate the unfortunate Scot. The alert light crew went to a full blackout just as the sword came down, and the scene ended with a dull "thunk" in the darkness as the sword hit the stage. The crowd, as well as the crew and cast backstage, went berserk with applause. When asked later what he would have done if the blackout hadn't happened, the actor playing MacDuff replied, "Well, I probably would have decapitated him. It's OK - he a method actor anyway."