Warning: Simpsons spoiler ahead

From the NY Post gossip page:

The [upcoming] episode will also feature "Lord of the Rings" star Sir Ian McKellen, the paper said.

The family goes to see him perform "Macbeth," but bring him bad luck by saying the play's name aloud - in defiance of theatrical superstition.

This leads to the actor being hit by scaffolding and getting struck by lightning.

As Lord Admiral Nelson would say: "Haw-haw"