It snapped a six-game winning streak for the Saints, who fell to 7-2. And it snapped a six-game losing streak for the Falcons, who are now 2-7.
How rare was the upset?
New Orleans was favored by 13.5 points, according to Caesars Sportsbook -- making the Saints the biggest favorite to lose a game this season.
It was the fourth time -- and the first since 2001 -- that a team on a losing streak of at least six games beat a team on a winning streak of at least six games, according to research by the Elias Sports Bureau. The others came in 1973 and 1948.
It was the first time since 2003 that a team with a record of 7-1 or better lost to a team with a record of 1-7 or worse, according to ESPN Stats & Information research.
It snapped the Saints' streak of 110 home games in which they reached double-digits scoring. They were one shy of the NFL record set by the Indianapolis Colts from 1997 to 2011.
"That's not us. We're better than that," said quarterback Drew Brees, who was particularly disappointed that the Saints went 3-of-12 on third downs and 0-for-3 on fourth downs.
Brees also tied a career high by being sacked six times -- against a Falcons team that entered the game last in the NFL with only seven sacks on the entire season.
And the Saints repeatedly killed their own drives or kept the Falcons' drives alive with a total of 12 penalties for 90 yards.
"We didn't play well enough to win today -- really against anybody," Saints coach Sean Payton said. "I mean, you pick a situation, and there's a good chance they won that situation.
"It's not a lesson, but this league is too good week-to-week where you can go in and not be focused and ready. This has always been a game played with emotion, and I thought they outplayed us.
"They outplayed us, they outcoached us, and they deserved to win."
Even more surprising was the fact that New Orleans' offense was expected to be even more explosive with running back Alvin Kamara and tight end Jared Cook returning from injuries following their Week 9 bye.
Yet somehow the Saints got routed on a day when they couldn't get out of their own way.
"You gotta beat the man in front of you, and it's pretty hard to do that when you're stepping on your own feet," Saints defensive end Cameron Jordan said.
The Saints' costliest penalty was probably a roughing-the-punter call against safety J.T. Gray when the Saints were trailing just 20-9 with nine minutes left in the fourth quarter. But they also got flagged three times when they allowed Atlanta to stay on the field for a 17-play touchdown drive in the first half.
Lattimore was shadowing Falcons receiver Julio Jones before he left the game in the second quarter with a thigh injury. He never returned despite trying to work himself back into shape on the sideline.
Before Lattimore's injury, Jones was held without a catch. Afterward, Jones caught three balls for 79 yards.
"They came in here in a hostile environment and made us like what they were serving, basically. So hats off to them," Saints defensive tackle Sheldon Rankins said. "But for us, it's back to the drawing board. We've gotta be able to rebound. This is in no way, shape or form a panic-button type of deal."
The Saints have three more division games upcoming on the slate, starting with a trip to Tampa Bay in Week 11.
"Shame on us if we can't get the corrections made," Payton said. "It's much easier to come in and watch tape after you win. But that's one of the things about this game that creates a little bit of toughness and grit."