Is there ever an acceptable reason to violate a child custody agreement?

When a parent files for contempt because the other parent has broken the child custody agreement, the parent is given the opportunity to explain why they went against the agreement. This leads me to believe that there are reasons the judge would find acceptable for violating the court order. What would those reasons be? And do those reasons apply when you have had time to try to get emergency custody or modify the existing order but have not tried to do so? I'm having trouble finding this answer