To tell or not to tell

I was on the bus on my way to work, when I noticed that the lady in front of me was wearing her top inside out. She was dressed in a smart suit and was obviously heading into work. She had taken her jacket off due to the heat hence revealing her inside out top.

I considered telling her, but changed my mind. After all I was a complete stranger and she may not have appreciated the comment. Since then I have been wondering whether I should have mentioned it. If the roles had been reversed, I would prefer to know, even if a stranger had to tell me. What is the etiquette? When is it ok to point things out to a stranger? What if it had been something less innocuous like an open fly or a revealed nipple? Is it more appropriate if it is more embarrassing?

Photo originally uploaded by tavekapa.