The shock of the season is a category that needs regular updating. First it was West Bromwich Albion's win at the Emirates Stadium, then Blackpool's triumph at Anfield. Now it is Sunderland's comprehensive victory at Stamford Bridge. Throw in Newcastle's 6-0 demolition of Aston Villa and Wolverhampton Wanderers' defeat of Manchester City and there is plenty of competition for the tag.
But Sunderland's case is strong. A club with 10 wins in their previous 108 Premier League games on the road must travel with rather more hope than expectation.
A team that had not conceded a league goal at home since March, as Chelsea had not, and with 31 wins and a solitary defeat in 35 matches, as they had at Stamford Bridge, must deem their ground nigh-on impregnable.
Instead, Sunderland stormed it, a feat that was more remarkable because only one of Steve Bruce's side - striker Asamoah Gyan - entered the game with a goal to his name this season.
The sight of Nedum Onuoha waltzing through the Chelsea defence to score the implausible opener gave it the feel of a freak result. That Carlo Ancelotti was missing much of the spine of his side, in centre-backs John Terry and Alex and central midfielders Michael Essien and Frank Lampard, can appear an explanation for an aberration.
Yet it is part of a general trend. With the chinks in their armour ever more apparent, the big beasts are being felled by the slings and arrows of their smaller brethren on a more regular basis.
Only Manchester United have not been floored, but the draw specialists have stumbled with such frequency that they sit uncomfortably in third place. This is their longest unbeaten start to a season under Sir Alex Ferguson, but United are the unconvincing "Invincibles" at the moment.
United retain that invaluable asset, their never-say-die spirit, which brought them a point on Saturday, but they were unable to overcome an Aston Villa side forced to field their seventh and eighth-choice central midfielders, in Barry Bannan and Jonathan Hogg.
Perhaps the fearlessness of youth, epitomised by the excellent Marc Albrighton, accounts for Villa's daring. Perhaps it is part of something bigger.
Because Sunderland, who remain unbeaten after facing the two Manchester teams, Liverpool, Arsenal, Tottenham Hotspur and Chelsea, fielded two strikers at Stamford Bridge.
This was no smash-and-grab raid, but part of a bold belief they could win. West Brom played Arsenal at their own game, fluent, passing football, when they won at the Emirates. Blackpool fielded three forwards for the entirety of their victory at Anfield (and, for good measure, replaced a full-back with a winger at half time).
They are in the vanguard of an attack on the established powers. The days when Arsenal could complete the season unbeaten (in 2004) or Chelsea could secure a Premier League record of 95 points (in 2005) seem distant.
But so, too, does 2009, when United ended with 90 points. It was the fourth successive season when the "Big Four", with crushing predictability, occupied the top four positions.
Now the era of invincibility is over. If there is little surprise in the identities of the division's leading quartet, the shocks are occurring on a weekly basis.
They are providing the best kind of sporting drama, where uncertainty is fused with disbelief, where heroes, whether Onuoha or Blackpool's Luke Varney and Marlon Harewood, come with an air of improbability.
With routine wins rarer, the fault lines in the leading teams are more visible and the rest are given greater encouragement. That the best are starting to look more fallible bodes badly when the Champions League becomes more serious, in February, but well for the domestic division.
It is the course of an impromptu rebranding from the self-styled best league in the world to the most competitive.
The division's unpredictability can render last week's tactical masterstroke this week's mistake. Two selections that had proved effective backfired when repeated on Saturday.
Raul Meireles had enjoyed perhaps his best game for Liverpool as an ersatz right-sided midfielder in the victory over Chelsea, but was lost in the same role in the defeat to Stoke City.
Sam Allardyce, meanwhile, had stifled Newcastle by fielding a five-man defence at St James' Park as Blackburn Rovers won. The same ploy failed at White Hart Lane, with Tottenham scoring four times and Allardyce swiftly reverting to four at the back.
Liverpool's Saturday consisted of destruction by Delap; Rory Delap, Stoke's long-throw specialist, had lost some of his lustre as opponents formed a method for nullifying him (Nemanja Vidic provided a recent master-class). But Liverpool were troubled time and again before his missile from the touchline led to Ricardo Fuller's opening goal and Roy Hodgson's subsequent comment that Liverpool had dealt with the long throw well until that point was the sort of assessment that undermines his own credibility. As he started to do two years ago, Delap loomed large in their imaginations before they even faced him.
Spare a thought for Birmingham City. A display of resolute organisation and determined defending earned them a point few predicted at Manchester City. Their reward? To drop into the relegation zone.