Manchester City slumped to a first defeat in 19 Premier League games as injury-hit Norwich deservedly won 3-2 at Carrow Road.
The champions had last lost in the league in January – a 2-1 reversal at Newcastle United – but the Canaries were in inspired form to deny Pep Guardiola's side.
Norwich, who were without seven players and could not call on City loanee Patrick Roberts, took the lead in the 18th minute.
Kenny McLean headed home a corner at the near post and the Canaries were soon 2-0 up when Teemu Pukki teed up Todd Cantwell to tap into an empty net.
City, who are already five points behind leaders Liverpool with just five games played this term, got themselves back in the game before half-time when Sergio Aguero nodded home Bernardo Silva's cross.
Yet the visitors' disastrous day took another turn when Nicolas Otamendi lost possession 20 yards from his own goal, and Emiliano Buendia set up Pukki to add a third in the 51st minute.
City pressed a resolute Norwich defence for the remainder of the game, finding an 87th-minute consolation through Rodri, but the hosts held on for a famous victory.
Norwich goalscorer Todd Cantwell, talking to BBC Sport, said: "We all got given our chance last season and we're paying the manager back by keeping his faith in us now.
"Me and Teemu have got a brilliant connection so if we can keep working well together and setting each other up and putting them away, we should be alright.
"It's unbelievable. It's an unbelievable feeling, we just want to kick on now and go again next week."
"Of course it is special day for us and for the club," added Norwich manager Daniel Farke.
"We had to be special in our plan for City because they are the best team in the world and we had setbacks too with our injuries, but we deserved the win today. We were exceptional."
Manchester City manager Pep Guardiola insisted he still "loves" his players despite the setback at Carrow Road, the second time City have dropped points this term after last month's 1-1 home draw with Tottenham Hotspur.
He claimed the odd slip-up is the norm rather than the exceptional form City have demonstrated in winning back-to-back Premier League titles.
"I know what we are as a team, what we have done, what we are going to do again, and I love my players. It is a pleasure and honour to be with them," said the Catalan.
"That is the challenge in a high level. The people cannot expect all the time [to] win or make 100 points, 90 points.
"It shows how difficult everything [we have done] is. Now we need to recover, go to Europe [in the Champions League] and come back."
For all of Norwich's injury woes, it was a City absence that appeared the most significant as Aymeric Laporte was badly missed at the heart of the visitors' defence.
The French centre-back will be sidelined until at least the new year after undergoing knee surgery and John Stones and Nicolas Otamendi did little to settle City nerves that they will be able to hold the fort until the transfer window opens again or Laporte returns in January.
Laporte's absence could be vital in Liverpool's quest to end a 30-year wait to win the league, but Guardiola warned the European champions that it is still early days in the title race.
"Sometimes [players] make mistakes," added Guardiola of his defence. "We will look to see what I can do to help them.
"We know the quality of our opponents, not just Liverpool, but especially Liverpool and I know five points is five points, but we are in September."