The theater is rife with superstitions, not the least is the one that says it’s bad luck to whistle backstage.
This has its roots in the fact that in the distant past, sailors were hired to run the fly loft on the premise that their expertise with knots would serve them well.
They communicated by a system of whistles. Therefore, someone else whistling could send wrong cues and create a bit of havoc.
View the discussion thread.