In Shakespeare's "Hamlet" the main character, Hamlet, fakes his own madness, in order to execute his revenge plans.

There is proof when he talks to Horatio, he seems perfectly sane, as he can trust his closest friend. However, around others he fakes madness, so as to not spoil his plot to kill Claudius.

Some may argue that when only Hamlet sees the ghost within his mothers chamber, this proves his madness, however, the ghost may have chosen to only be seen by Hamlet. His mother was already freaking out enough, and the ghost may have pushed her over the edge.