Wigan Athletic pulled off the big shock of the FA Cup fourth as the team from the third-tier of English football knocked out Premier League side West Ham 2-0 with a famous double from their cult hero Will Grigg.
West Ham were not the only top-flight side to flounder in England's famous knock-out tournament, however, as Tottenham Hotspur were eight minutes from tumbling out at League Two (fourth-tier) Newport County until a Harry Kane tap-in salvaged a 1-1 draw.
The Premier League's bottom club Swansea City were also held to a 1-1 draw at League Two Notts County while Huddersfield Town drew 1-1 with Championship (second-tier) visitors Birmingham City.
It was David Moyes' West Ham who suffered the sort of nightmare Cup afternoon the bigger clubs dread.
Wigan's hero was the Northern Ireland striker Grigg, who again found himself the subject of the fans' favourite chorus of "Will Grigg's On Fire" as he headed home after seven minutes and converted a second-half penalty.
To compound their misery, the Hammers had to play for more than 40 minutes with 10 men after Arthur Masuaku was sent off soon after the break for spitting at Wigan's Nick Powell.
The League One leaders had knocked out Premier League Bournemouth in the third round and Moyes quickly recognised why when Grigg sent a powerful header past England goalkeeper Joe Hart.
Wigan's fortunes have dipped since lifting the Cup in 2013, the year they earned the unwanted distinction of being the only side to win the trophy while also being relegated from the top flight.
Yet their fans at the DW Stadium, who have recently been used to their side creating FA Cup shocks, were left in ecstasy when Grigg scored his second in the 62nd minute from the spot after a hand-ball from Reece Burke.
"A Cup run is great. The extra games are no problem for us. If it is a top Premier League team next, we will take that," said a delighted Grigg, after becoming the tournament's top scorer this season with his fifth and sixth goals.
Newport were close to pulling off an authentic FA Cup sensation against a side 72 places above them in the league hierarchy as they led Spurs 1-0 through a first-half headed goal from Irishman Padraig Amond.
Yet the dream evening for the Welsh club in front of a 10,000 sell-out at Newport's tight Rodney Parade ground was ruined when Kane pounced from close range after an 83rd-minute corner for his 30th goal of the season in all competitions.
It was heartbreaking for Newport, who had held out courageously in the second half, but their substantial consolation is a replay at Wembley Stadium.
Swansea were also given an uncomfortable ride in a 1-1 draw at Meadow Lane after Luciano Narsingh's goal just before halftime was answered by another goal in the 62nd minute from Notts County's own cult figure, 34-year-old Jon Stead.
Stead, who helped knock out Jose Mourinho's Chelsea in 2015 while at Bradford City, has now scored in all four rounds of the Cup this season.
Huddersfield were in control at home to Birmingham through a first-half Steve Mounie goal but now have to go back to St Andrew's after a 54th-minute Lukas Jutkiewicz equaliser.
Coventry City, who caused a sensation in the third round by knocking out Premier League Stoke City, continued their heroics with a 1-0 win at League One MK Dons to ensure that one League Two side will be in the fifth round.
In the day's early game, Leicester City made light work of a potentially troublesome hurdle as doubles from Kelechi Iheanacho and debutant Fousseni Diabate eased them towards a 5-1 victory at League One Peterborough United.
The first all-Premier League tie of the round saw Southampton knock out Watford 1-0 with a fourth-minute goal from Jack Stephens - the perfect way for the defender to celebrate his 24th birthday.
Nottingham Forest, who knocked out holders Arsenal in the third round, had their adventure ended in a 2-1 defeat at fellow Championship side Hull City.