It's one of the best-remembered images of the Vietnam War: A helicopter perched precariously atop a small rooftop at the U.S. embassy in Saigon. Desperate South Vietnamese men and women crowding a ladder trying to reach it. A single C.I.A. employee standing between the aircraft and the throng, his arm outstretched. The picture, taken on April 29, 1975, became an instant—and enduring—symbol of America’s humiliating defeat in the controversial conflict.

There’s just one problem with this widely shared memory: It wasn’t actually the U.S. embassy, but another building a half-mile away.

The iconic photo was the work of Hubert Van Es, a Dutch photojournalist who’d been covering the war since the late 1960s. He was working for the United Press International (UPI) wire service at the time and was in its offices in the South Vietnamese capital when he saw the chaotic scene from a few blocks away.

“Like so many things about the Vietnam War, it’s not exactly what it seems,” Van Es wrote in a 2005 remembrance for The New York Times. “In fact, the photo is not of the embassy at all; the helicopter was actually on the roof of an apartment building in downtown Saigon where senior Central Intelligence Agency employees were housed.”

Beginning on April 29, 1975, as North Vietnamese troops were advancing toward Saigon, Americans launched Operation Frequent Wind, the largest helicopter evacuation operation in history. Over some 24 hours, choppers landed at 10-minute intervals at or near the U.S. embassy, eventually ferrying more than 7,000 Americans and Vietnamese away from the soon-to-be-occupied capital.

Many non-Vietnamese journalists had already left Saigon, Van Es recalled, and he was the only UPI photographer who remained. As a Dutch citizen, he believed he was in less danger.

After a morning spent covering the unfolding events around the city, he’d returned to the UPI bureau on the top floor of the Peninsula Hotel to develop and print his pictures. At about 2:30 p.m., a fellow staffer alerted him to the commotion on the roof of the Pittman Apartments, about four blocks away.

“I grabbed my camera and the longest lens left in the office—it was only 300 millimeters, but it would have to do—and dashed to the balcony,” Van Es wrote.

Van Es fired off about 10 shots before returning to the darkroom to process his film and write captions in time for the UPI’s regular 5 p.m. photo transmission to Tokyo. “For the caption, I wrote very clearly that the helicopter was taking evacuees off the roof of a downtown Saigon building,” he later remembered. “Apparently, editors didn't read captions carefully in those days, and they just took it for granted that it was the embassy roof, since that was the main evacuation site.”

Van Es finally left Saigon, now renamed Ho Chi Minh City, on June 1, 1975—“invited” to do so by the new regime, he noted. He went on to cover other conflicts, including the Soviet invasion of Afghanistan, before his death in 2009.

But that single photo may be his most lasting legacy—despite the lingering mix-up over precisely what it showed.

“This mistake has been carried on in the form of incorrect captions for decades,” he lamented in 2005. “My efforts to correct the misunderstanding were futile, and eventually I gave up.”