The example Franzl gave is one where ignorance is no excuse too. It's easy enough to look up online where you can legally have a 15 year old blow you.
Age of consent and statutory rape laws are a perfect example of what I'm talking about. If a young couple were to travel across the country, in some states, their having sex would be perfectly acceptable, but in others, he's technically raping her. It's not their responsibility to go out of their way to inform themselves of such silly restrictions; it's the state's responsibility to publicize how their laws differ from others'.
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Actually, a decent philosophical case could be made that the phrase is unacceptable even for major crimes.
Let's say that a country exists somewhere that allows people to murder each other if, say, the victim insulted the aggressor's wife.
A citizen of that country, born and raised and with no experience of other cultures, decides to visit the U.S. The only legal requirements he needs are a passport and a visa; he doesn't need to take a test so that he's familiar with our laws and customs.
Before long, a New York cabbie makes a disparaging remark about our fellow's wife, so he kills him. Obviously he has committed a crime within our territory, but he
didn't know that. Why would he have known it? A reasonable defense could be made on that basis.