Spurs had been coasting to what appeared the most straightforward of victories on England's south coast on Sunday, having outplayed Brighton at the Amex Stadium with some scintillating stuff to be 2-0 up at halftime though goals from Brennan Johnson and James Maddison.
But demonstrating the inconsistency that's become a Spurs hallmark - and not just under Postecoglou - the Australian's team were almost unrecognisable after the break, folding so meekly that they conceded three goals in 18 minutes to Yankuba Minteh, Georginio Rutter and Danny Welbeck.
Spurs' striker Dominic Solanke rues a missed chance in the Brighton loss. (AP PHOTO)
It was the 10th time that Spurs have lost a game in the Premier League era when they were two goals ahead. No other club can match that dispiriting stat.
Postecoglou has been occasionally tetchy and often defiant when Spurs' performances have been questioned this season, but this time his response on Sky Sports, with head bowed, was unusually subdued.
"That's disappointing. I'm frustrated, absolutely gutted, that's probably the worst defeat we've had since I've been here," he said.
"Unacceptable second half, we weren't anywhere near where we should be, and we just got carried away with how we were going.
"We kind of accepted our fate, which it's a bit hard to understand, because we haven't done that since I've been here. We've usually fought for everything, but when you don't, you pay a price, and we paid the price.
"Maybe things were travelling on too smoothly, and football and life will trip you up if you get too far ahead of yourself - and that's what it looked like to me second half.
"There's no message (to the players). It's a terrible loss for us, as bad as it gets, and there's only one way to fix it - and that's my responsibility."
What must have been particularly galling for the 59-year-old was that, after a sticky start to his second season in charge at Spurs, they had appeared to be moving into a higher gear, reeling off five straight wins including an impressive 3-0 win at Manchester United last week.
With Spurs having slumped to ninth in the table when they could have been sixth with a victory, many of Postecoglou's squad will now disappear on international duty with food for thought.
"So they will process individually, and then 'll deal with it when everybody gets back," said the manager.
Spurs' England international Maddison also admitted on Sky that they had "lost complete control of the game."
"In adversity in the Premier League, the best teams stay strong in that moment and weather the storm," he said. "We definitely didn't do that."
