The theater is known for having many superstitions, including telling actors to "break a leg" instead of wishing them good luck, and referring to Shakespeare's "Macbeth" only as "The Scottish Play." But superstitious actors also avoid whistling because, in the early days of theater, doing so could result in serious injury from a confused stagehand.
Theaters used to employ off-duty sailors to run the rope systems because they were similar to riggings on ships. Much like they would on a ship, the stagehand sailors used a series of whistles to indicate changes in the scenery. An unplanned whistle from anybody else in the theater could send the scenery flying, potentially causing injury or worse. Although the riggings have changed and experienced stagehands have replaced sailors, the superstition has persisted.