The instructions of how to pitch on the Wiffle ball box are wrong...well sort of.  The box always featured diagrams of how to throw a curve and slider, but according to Nathan Cobb of the Globe, the diagrams are wrong.  Mullany the Elder(founder), a left-hander, showed a graphic artist how he gripped the ball.  The artist then drew a right hand, while keeping the ball in the same position.  So the diagrams were backwards, and they were never changed.