At half-time, all the headlines looked like they would be about Tottenham winger Brennan Johnson continuing his superb run of form.
Having scored five goals in his first 38 games for Spurs, the 23-year-old Wales international has now scored in each of his past six games in all competitions.
He opened the scoring at the Amex when he timed his run perfectly to connect with Dominic Solanke's through ball and calmly slotted past home goalkeeper Bart Verbruggen.
After James Maddison added a second, Johnson had a chance to get a third for Tottenham just before the break, but shot over after racing through following Dejan Kulusevski's pass.
But that two-goal lead was as good as the afternoon got for Tottenham, as they somehow threw away control of the match as Brighton scored three times in an 18-minute spell in the second half.
Boss Ange Postecoglou called the collapse "probably the worst defeat since I've been here", although the impressive form of Johnson did at least provide one positive for Spurs.