Even taking into account this rather unusual Premier League season, West Brom were not expected to get anything from their trip to Chelsea.
Beyond just their respective positions at opposites ends of the table, the visitors went into the fixture having managed just one away win all season in the league.
Stamford Bridge had certainly not been a happy hunting ground either – the Baggies had not triumphed there since September 1978 prior to Saturday's trip. They went on to finish third that season, as well as reaching the last eight of the UEFA Cup. Now, they just hope to stay in the top flight.
Chelsea, meanwhile, were unbeaten under Thomas Tuchel. Not only that, they had not conceded in their five Premier League home games since the German was appointed as Frank Lampard's replacement. The issues of inconsistency under the previous regime appeared to have been fixed.
The form guide pointed to only one outcome, yet the early kick-off produced a stunning scoreline: Chelsea 2 West Brom 5 (FIVE).
Here's to you, Mr Robinson
There was little sign of what was to come when Christian Pulisic put the Blues in front with his first league goal in the calendar year. However, less than two minutes later and the home team were down to 10, Thiago Silva becoming the oldest player to be sent off for Chelsea in the competition at 36 years and 193 days, the Brazilian shown a second yellow card for a foul on Okay Yokuslu.
Matheus Pereira scored twice in first-half stoppage time to turn the game around and put West Brom in front at the break. Sam Johnstone was the unlikely provider for one of them, in the process becoming the first West Brom goalkeeper to assist a Premier League goal.
There was to be no turnaround after the teams had turned around, either. Callum Robinson helped himself to a brace, making him the first player in Premier League history to score at least twice in home and away games against Chelsea in the same season - he had also struck twice when the sides drew 3-3 at the Hawthorns back in September.
All of the forward's top-flight goals have come against Chelsea, having also managed to score against the Blues last term when playing for Sheffield United.
A Mbaye Diagne effort was sandwiched in between Robinson's impressive double, meaning West Brom hit five away from home for the first time since February 2012. They remain deep in relegation trouble, of course, but the suddenly bouncing Baggies will hope they can use this result as a springboard for the most unlikely of escape acts.
Tuchel left stunned as Big Sam does it again
A day after being named Premier League Manager of the Month for March, Tuchel saw his unbeaten record come to an unexpected end. He had been unbeaten in 10 league games at the start of his reign, while Chelsea had won their previous 11 fixtures when hosting opponents sitting inside the relegation zone, doing so by an aggregate score of 31-4.
West Brom did not appear to be a difficult hurdle to clear, but the challenge became much tougher once Silva - making just his second appearance since February 4 - was dismissed.
Tuchel watched on as the Blues shipped five goals for just the second time at home in the Premier League era. The other visiting team to achieve the feat were Arsenal, when Robin van Persie's hat-trick helped the Gunners run out 5-3 winners against a Chelsea line-up that included club legends John Terry, Petr Cech and Lampard, while a young Romelu Lukaku made an appearance off the bench.
"I did not see that second half coming," Tuchel said afterwards, seemingly speaking on behalf of everyone who had sat through proceedings. In a case of stating the obvious, he also admitted his team "didn't defend well".
Something about West Brom seems to trouble Chelsea, though. They have conceded eight goals in the two meetings in 2020-21, the most they have shipped against one side in a single campaign in the competition, in collecting a solitary point.
As for Tuchel's opposite number, Sam Allardyce becomes the first visiting manager to win at Chelsea with three different clubs in the Premier League, having previously done so while in charge at Bolton Wanderers and Crystal Palace.
What made the result even more astonishing is West Brom had scored just 10 goals in 16 league games under the former England boss beforehand, averaging out at 0.6 per game, and had drawn a blank in their three outings prior to the international break in March.