Sisters of Mercy sin
Refers to an act of ordinarily-frowned-upon nature, but which in extreme circumstances could be overlooked or even approved of. Comes from the nuns' surreptitiously ripping out the plug-wires and spark-coils from the
Gestapo's cars so that they couldn't pursue the von
Trapp family in "The Sound Of Music"; the emotionally-shaken-but-still-demure habit-clad heroines then ask their
Mother Superior for forgiveness, which it can be assumed that she readily gives, considering the horrid fate that would likely have befallen the von Trapps had they been captured.