Ian McKellen (character)

Sir Ian McKellen is a famous English actor. His career spans four decades, but in recent years he is best known for playing Gandalf in the Lord of the Rings movies and Magneto in the X-Men movies. The Simpsons met McKellen when they were visiting England.

History
McKellen was starring in the Shakespeare play Macbeth and was standing outside the theatre when the Simpsons came walking by. After they exchanged greetings, McKellen said he had a show to do, and said that in the theatrical world, the custom was to not mention the name of the play in which he was appearing, in order to avoid bad luck. Marge, seeing the play's name on the marquee, innocently asked, "Macbeth?" and a bolt of lightning struck McKellen. A round of conversation ensued, during which everyone (including McKellen) said "Macbeth" at least once, and each time McKellen was struck by lightning. Bart then deliberately said the play's name three times in succession, and McKellen was quickly zapped by three lightning bolts.

Finally, after McKellen had been struck several times, Marge apologized for what had happened. McKellen replied that it was all right because they didn't know. He then excused himself, saying he had a perfomance to give. The Simpsons wished him good luck. McKellen, aghast, said that was also bad luck, and a piece of the marquee promptly fell on him.

Behind the laughter

 * The conversation about Macbeth, during which McKellen is repeatedly struck by lightning, is a reference to the theatrical superstition about mentioning the play's name being bad luck.
 * There is also a superstition in which wishing someone "good luck" before a performance will bring them bad luck. Among actors and musicians, the custom is to instead say "break a leg" when wishing a performer good luck.