At half-time at the Amex Stadium, Tottenham were comfortably on course for a sixth straight win in all competitions and heading to within a point of the Premier League top-four.
Goals from Brennan Johnson – his sixth in as many games – and James Maddison had put Ange Postecoglou’s side in total control against wide-open Brighton.
By full-time, however, Spurs were facing an uncomfortable inquest into a stunning second-half collapse, which saw the Seagulls score three times inside 21 minutes of the restart.
The first thing to acknowledge is that Brighton were outstanding after the interval and their young coach, Fabian Hurzeler, deserves huge credit for turning the game on its head.
The introduction of Pervis Estupinan for the struggling Ferdi Kadioglu at the break nullified Spurs’ threat down the right, helping to transform the feel of the contest.
For Tottenham, though, this was another alarming loss of control from a seemingly serene position of strength.
Once Brighton had halved the deficit through Yankuba Minteh, the visitors were visibly rattled, their composure suddenly shot to pieces.
The usually-unflappable Micky van de Ven was at sea for Brighton’s first two goals, while Destiny Udogie was poor for all three in what was surely his worst display in English football.
Pedro Porro was run ragged by the outstanding Karou Mitoma, and Cristian Romero never looked comfortable against Welbeck, even in the first half.
Their capitulation was about more than individuals, though, and felt like another example of Tottenham's inability to control matches and tendency to be spooked by setbacks.
Postecoglou was actually asked by Sky Sports before the game how Spurs could keep it “controlled” on the South Coast.
"We don’t,” he replied. "Let’s keep it open, that way we entertain everyone and hopefully get the result we want."
It is all well and good wanting to attack and no doubt Spurs are entertaining for neutrals, but they needed to do the opposite when they emerged with a two-goal lead for the second half.
A top team would likely have aimed to slow the game down, perhaps keep it tight for a spell and try to gradually put out any last embers of optimism in the home crowd.
Instead, Mitoma escaped from Porro within two minutes of the restart and Minteh finished at the back post after Van de Ven and Udogie made a hash of the Japan winger’s cross.
From there, it was one-way traffic, with a shell-shocked Spurs increasingly jittery and struggling to compete physically, as Brighton sensed blood.
There is, plainly, a time and a place to take risks and play on the front-foot, but 2-0 up away from home in the Premier League are not among them.
This game felt like a more extreme example of the opening day of the season, when Spurs were comfortable and in front against Leicester City but ended up being a little fortunate to escape with a 1-1 draw after losing their cool after the break.
One big problem is that Postecoglou’s team will always give up chances and in all five of their recent wins on the bounce – against Coventry, Brentford, Qarabag, Manchester United and Ferencvaros – there were moments when they might have shipped a careless goal (or another, in the cases of Coventry and Brentford).
Postecoglou, who afterwards described his players' performances as "unacceptable", was not blameless, proving unable to help his side wrest back the momentum in the second half. The Australian did not make a substitution until the 78th-minute, introducing Yves Bissouma and Pape Sarr long after the damage was done.
It was obvious long before then that Brighton were in the ascendency and Postecoglou arguably needed to do something to help Spurs re-establish a foothold.
He was without Son Heung-min and Richarlison and had four teenagers on the bench but midfielders Bissouma and Sarr might have been able to make a positive difference sooner.
Spurs now head into another international break on the back of a damaging defeat – just as they did last month following the north London derby – and leaving Postecoglou with plenty to consider.
The manager knows his side are still learning his approach and developing as a collective but, for all their recent strides forward, this was a sobering reminder that Spurs remain a work in progress.
