Justice Antonin Scalia’s factual error in a dissenting opinion Tuesday has become the talk of the legal community as experts puzzle over the extraordinary nature of the Reagan-appointed justice’s blunder, which the Supreme Court quietly corrected as of Wednesday morning.

It’s common for the Supreme Court to make typographical corrections and insubstantial edits to a decision after its release. But it’s exceedingly rare to see a factual error that helps form the basis for an opinion. Legal experts say Scalia’s mistake appears to be wholly unprecedented in that it involves a justice flatly misstating core facts from one of his own prior opinions.

“This is a topic I know fair amount about, and I do not know of any other instance when a Justice has mischaracterized one of his own prior opinions, let alone in such a loud fashion and when he is otherwise criticizing others for their blunders,” said Richard J. Lazarus, a Harvard law professor. “I strongly doubt it has ever happened before.”

“I have seen sloppy footnoting before but nothing this bad,” said Garrett Epps, a constitutional law professor at the University of Baltimore.

Dan Farber, a law professor at the University of California, Berkeley, called it “an unusually major mistake, and all the more surprising because Scalia wrote the American Trucking opinion” that he mischaracterized in Tuesday’s ruling.

Scalia was dissenting from a 6-2 decision upholding the Environmental Protection Agency’s authority to regulate cross-state coal pollution. To help back up his judgment, he cited a 9-0 opinion he wrote in 2001 called Whitman v. American Trucking Association. But the EPA’s stance in that case was the exact opposite of what Scalia said it was in Tuesday’s opinion.

It’s not the first time a justice has erred. In 2008, a majority opinion written by Justice Anthony Kennedy mischaracterized federal law when ruling unconstitutional the death penalty for raping a child. His opinion argued that the death penalty for such a crime existed in just six states and not on a federal level. That claim was false, as the New York Times reported afterward: Congress had passed a law two years earlier saying child rape was subject to the military death penalty. The Court subsequently issued a corrected opinion with a footnote noting the change, unlike this week when its correction came surreptitiously and without notice.

Lazarus said Kennedy’s error was “very different” as it was prompted by a misrepresentation of federal law by the U.S. Solicitor General, on whom the Court “should be able to rely” for such matters. “That was the Solicitor General’s mistake, more than the Court’s,” he said.

Epps recalls that the error in Kennedy’s opinion was “a big deal in constitutional law circles” but not as bad as Scalia’s mistake this week: “It’s almost as embarrassing not to know federal law as it is not to know your own opinion.”

Scalia, 78, is the longest-serving member of the Supreme Court, appointed by Ronald Reagan in 1986. His dissenting opinion in Environmental Protection Agency v. Homer City Generation was joined by Justice Clarence Thomas.

“Of course, we all no doubt make mistakes in what we write, including big ones,” said Lazarus, who told TPM he formally notified the Supreme Court of Scalia’s mistake and suggested revisions. “But, our stuff is not subject to the same level of scrutiny because we are not, after all, Supreme Court Justices.”