In 2003 (sometimes these case take a while to unfold!), a federal air marshal received a text that TSA was " cancelling all overnight missions from Las Vegas until early August." Just days earlier, the TSA had briefed the marshal about terrorist plots to hijack passenger flights. So, believing that cancelling the flights was dangerous and illegal, the marshal contacted a reporter and disclosed the cancellation of the missions. The TSA found out and fired him.
The marshal claimed the TSA violated federal statutory whistleblower protections under 5 U. S. C. §2302(b)(8)(A). The government claimed that the firing fell under an exception to the whistleblower protection for disclosures "specifically prohibited by law." The "law" they relied upon was a TSA regulation that prohibited disclosure of "sensitive security information."
Bottom line? The Court held that a regulation did not count as a "law." Therefore, even if the marshal violated the regulation, he did not violate the law. Therefore his activity was protected by the whistleblower statute.
Fun sidenote: Justice Sotomayor (joined by Justice Kennedy) authored a dissent in which she cites Justice Scalia's book, Reading Law: The Interpretation of Legal Texts (co-authored by Bryan Garner). Nice plug, Justice Sotomayor!