The newly canceled bust-up of SEAN PENN’s marriage to long-suffering ROBIN WRIGHT PENN was even uglier than reported – when he suddenly demanded a divorce in April, she exploded and flung her wedding ring right at his head, say friends.

Robin was “completely blindsided” by Sean’s ultimatum because just a week before, she’d been singing the praises of their marriage to the press.

“Robin had no idea,” says a friend. “Sean had strayed before, and they’d even split for a while in 2007, but this time she thought they’d found their equilibrium and it was going to last.”

When Sean confronted her and delivered his “I love you, but this isn’t working” talk, which she’s heard before – then added that he saw himself settling down with someone new and “starting a family” – Robin screamed in fury, ripped off her ring and flung the thing, just missing his head.

Then she stormed out of the house as Sean bellowed: “You’re acting like a child!” Divorce is off-again, but… stay tuned.