The mischief makers on my keyboard are the I and O, because they're right next to each other, which yield such gems as Westmonster, and the Dike of Norfolk.

But my favourite was a poster for a lute recital which a friend was giving. He was good at calligraphy so he hand-lettered his own poster, which (after an entire night's work) elegantly annouced his forthcoming Lute Recital. Only problem was, he left the "I" out of Recital.