alt Hacker NewsRe: imposed by different jurisdictions. I don't know the letter of the law in the US, but here in Australia at least, most such differences are easily resolved by section 109 of our constitution (note: Commonwealth == Federal):
"When a law of a State is inconsistent with a law of the Commonwealth, the latter shall prevail, and the former shall, to the extent of the inconsistency, be invalid."
Hmmm, just googled, and it would seem that Article VI Clause 2 of the US constitution, aka the Supremacy Clause, is basically equivalent.
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You're assuming the contradiction is between a federal law and a state law and not e.g. a law of the US and a law of France.
One of the more recent fun absurdities is some jurisdictions claiming that if you have a service accessible over the internet then you're subject to their (i.e. every) jurisdiction.
In the US, the Supremacy Clause only holds insofar as federal law doesn't overstep the restrictions on federal power outlined elsewhere in the Constitution. The wording is "the Laws of the United States which shall be made in Pursuance [of the Constitution]".
If you follow federal law over state law, but then a federal judge holds that the federal law you followed was an unconstitutional overstep in a domain that the Constitution reserves to the states, you are SOL