Jannik Vestergaard scored in injury-time to secure a 3-3 draw for Southampton at Chelsea, who had led 2-0 shortly before half-time.
Timo Werner's brace, his first league goals for the club, looked to have Frank Lampard's side heading for a straightforward win.
But Danny Ings reduced the deficit in the 43rd minute and yet another error from Chelsea's erratic goalkeeper Kepa handed Che Adams an equaliser just before the hour.
The Blues restored their lead immediately though, with Kai Havertz also opening his Premier League account.
And with time running out Theo Walcott, making his second Saints debut almost 15 years after leaving the club as a teenager, was the provider for Vestergaard to fire home.
Another horror show for Kepa
What had seemed like a day to remember for Werner, vindicating his £53m transfer fee, was ultimately a day to forget for Kepa.
The out-of-form Spaniard's latest clanger proved costly, as he collided with the post when Adams scored his goal – and that after finding himself stranded from Kurt Zouma’s shy back-pass.
Prior to that Werner had stolen the show.
The Germany striker quickly had the ball in the net, heading in from a Ben Chilwell cross, only for the offside flag to deny him.
The pacy forward was not to be denied moments later however, conjuring a sumptuous turn off Chilwell’s smart pass to leave the struggling Jan Bednarek for dead. Werner then raced into the box, cut across the face of goal and still found time to add a fine finish.
For a maiden Premier League goal, this was something special – and entirely in keeping with Chelsea’s expectations of their big-money summer signing.
Werner doubled his and Chelsea’s tally just before the half-hour, racing onto Jorginho’s lofted pass and again catching Bednarek cold.
The former RB Leipzig man chested down, chipped over the onrushing McCarthy and then nodded into the empty net.
Chelsea were rightly in cruise control at 2-0, only for Havertz to be hustled off the ball deep in his own half.
Saints latched onto the chance, and the ever-clinical Ings delivered a fine finish. When Southampton levelled through Adams Chelsea only had themselves to blame.
Zouma’s under-hit back-pass left Kepa stranded, and the Spain keeper clanged against the near post amid a game of six-yard box pinball. Adams proved the sharpest amid the mayhem, latching onto the loose ball before drilling home in style.
Kepa’s shot at redemption only came about so soon due to Edouard Mendy’s thigh injury, and the 26-year-old will doubtless rue another missed opportunity to reassert his Stamford Bridge status.
The aggrieved Blues at least vented their clear frustrations in a quick-fire move to regain the lead.
This time Werner turned provider, laying the Blues’ third on a platter for Havertz, who dinked home neatly.
Saints refused to let Chelsea coast to the win, with Adams firing wide after smart approach play from Walcott, who impressed on his first Saints appearance in almost 15 years.
Kepa produced a good save to deny Ings, but Chelsea could not find the fortitude to hold on.
After a half-cleared free-kick, up popped Vestergaard with a low drive, that eluded everyone – to floor Chelsea and send Saints into wild celebrations.