Here is a bizzare, but completely true story regarding the flight crew and arbitrary "judgement":

I was travelling on a BA trans-atlantic flight with someone who is very anti-smoking -- hates anything to do with cigarettes. One of the FA's got it into her head that he'd been smoking in flight, and started to make a scene. My friend, correctly, resisted, and finally she backed off, though muttering that she knew she could "detect" tobacco on him. What if her purser had given her the benefit of the doubt?