|League One leaders Wigan, who won the competition in 2013, stunned Premier League West Ham to reach the fifth round|
West Ham were not the only top-flight side to struggle in England’s famous old knock-out competition as bottom club Swansea City were held to a 1-1 draw at League Two (fourth-tier) Notts County.
Premier League Huddersfield Town were also held 1-1 by Championship (second-tier) visitors Birmingham City but it was David Moyes’ West Ham who suffered the sort of nightmare afternoon that the bigger clubs dread.
Wigan’s hero was the Northern Ireland international striker Grigg, who again found himself the subject of the Wigan 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, who also suffered injuries, 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.
Having made six changes to his side for the visit to the League One leaders, Moyes must have recognised the worst once he saw Grigg send a powerful header past England goalkeeper Joe Hart as Wigan dominated the early stages.
Wigan’s fortunes have dipped since lifting the Cup in 2013, the year they also earned the unwanted distinction of being the only side ever 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 upsetting Premier League sides in the FA Cup, were left in ecstasy when Grigg scored his second in the 62nd minute from the spot after a hand-ball from Reece Burke.
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 humble Jose Mourinho’s Chelsea in 2015 while at Bradford City, has now scored in all four rounds of the Cup this season.
Huddersfield looked in control at home to Birmingham through a first-half Steve Mounie goal but will 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 at least one League Two side will be in the fifth round.
League One side Rochdale earned a 2-2 draw at Championship side Millwall, who needed a 90th minute goal from Ben Thompson to secure a replay.
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 side 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, saw their adventure end in a 2-1 defeat at fellow Championship side Hull City.