The Premier League champions were cruising to a comfortable success when they went 3-0 up against the Dutch side, and were still in a commanding position with 15 minutes remaining.
Erling Haaland netted twice, either side of Ilkay Gundogan's deflected volley, as City looked set to end their five-game losing run in style.
An error from Josko Gvardiol allowed Anis Hadj Moussa to race in behind and pull Feyenoord back into the game, before Santiago Gimenez closed the deficit to one goal shortly after.
The unthinkable then occurred, as an erratic Ederson came steaming out of his goal, missing the ball and Igor Paixao, who then crossed for David Hancko, and the Slovakian defender headed home the equaliser.
Pep brutally honest in post-match interview
Speaking to Amazon Prime Sport following the game, Guardiola was honest about the current morale of the team, in what has been a scarcely-believable run of form.
"We lost a lot of games lately, we are fragile, and of course we need a victory." Guardiola told interviewer Gabriel Clarke.
"The game was good for the confidence, we were playing at a good level and then for the first time, something happened, we have problems, yes.
"The three changes [subs at 3-0]? The game is never over, but at 3-0 I could not see any danger."
Guardiola singled out individual mistakes and a lack of quality for the capitulation, hinting that their problems could be more than a confidence issue.
"I don't know if it's mental, it's a question of techniques, the first goal cannot happen, and the second as well. We do well but we don't win games."
Guardiola explains visible cuts and scratches on his face
Then when Guardiola was in front of the media in his post-match press conference, he was asked about the cuts and scratches clearly visible all over his face.
"With my finger, my nail, here [down his nose]." Guardiola revealed.
The Man City's manager's deep frustrations were then laid bare when he got up to leave the press conference saying: "I want to harm myself."
After Feyenoord's first goal, Guardiola was seen with his head in his hands for a lengthy period, with the manager looking deeply stressed, despite his team not being in any immediate danger.
Those fears were soon realised though, with Feyenoord's quick-fire double shortly after that seeing City throw the game away in comical circumstances.