Vortigern and other hoaxes.

In the 1790s, William-Henry Ireland, the son of an antiquarian, perpetrated one of the great literary hoaxes when he managed to fool many of the contemporary literary with a range of "lost" documents which he purported to be in Shakespeare's own hand. They included letters, poetry and a whole play, Vortigern, which even received a premier with John Kemble. The Smithsonian has the whole story, including a blow-by-blow account of this major theatrical event:

Vortigern was obviously not a theatrical masterpiece, regardless of who had written it. The first hint of disaster came in the third act, when a bit player—a skeptic, like Kemble—overplayed his lines for laughs. The crowd grew more restive in the final act, when Kemble as King Vortigern addressed Death with mock solemnity:

O! then thou dost ope wide thy hideous jaws,
And with rude laughter, and fantastic tricks,
Thou clapp’st thy rattling fingers to thy sides;
And when this solemn mockery is ended—

The last line he intoned in a ghoulish, drawn-out voice, which provoked several minutes of laughter and whistling. Kemble repeated the line—leaving no doubt as to what mockery he was referring to—and the crowd erupted again. The performance might have ended there, but Kemble stepped forward to ask the audience to permit the show to go on.

The full text of the play with a preface by Ireland's father, who was still convinced of its authenticity years after his son had confessed, is available elsewhere on-line.