What is the ruling on a strange man seeing a woman's nakedness without her noticing?

Both men and women are required to cover their awrah (private parts). If someone sees it unintentionally or inadvertently, there is no sin upon them, as Allah Almighty says: "Our Lord, do not take us to task if we forget or make a mistake" (Quran 2:286), and the Prophet, peace and blessings be upon him, said: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are forced to do."