Since defeating Chelsea to reach the Semi-Finals in 2007-2008 Barnsley have won only 5 of 17 FA Cup matches (inc replays) 3 of which came in 2012-2013.
That season is also the only time since that memorable run 10 years ago that the Reds have advanced beyond the Third Round.
The 4-1 defeat at the Den is incidentally Barnsley's heaviest FA Cup defeat again since 2012-2013 when they crashed 5-0 at Manchester City in the Quarter-Final.
It is however The Reds heaviest FA Cup defeat against a team outside the top flight since the First Round of 2002-2003 when they lost 4-1 at home to Blackpool - Barnsley were a second tier side whilst Blackpool were in the third tier.
This defeat brought to an end Barnsley's five match winning run against Millwal, in which they had scored 12 goals.
It is only their second defeat in nine meetings (7 wins) and is the heaviest defeat against the Lions since a 4-1 defeat at the old Den in September 1990.
There has been 21 goals scored in the last five Barnsley/Millwall clashes (12 for Barnsley, 9 for Millwall)
Barnsley conceded three second half goals in a game for the first time since April (3-2 defeat at Bristol City)
For the first time in six outings there was a first half goal in a Barnsley game.
And like buses, you wait ages for one and then two come along at once, it was also the first time in six that two first half goals were scored.
It was however the first time both Barnsley and their opponents scored a first half goal in seven outings (the 3-1 defeat at Bolton on the 2nd December)
It is only the second time in 13 outings that both sides have scored a first half goal.
Brad Potts 11th minute effort was the earliest Barnsley have scored since Tom Bradshaw's 2nd minute goal against Birmingham on the 4th November.
Potts has scored three times for Barnsley now, all three have come away from home (at Preston, Burton and Millwall) and it is the first time the Reds have lost when Potts has scored.
Adam Hammill provided the assist for the goal, only his second assist of the season, and both have been for Potts.
Barnsley have opened the scoring in each of the last four Millwall/Barnsley meetings.
Joe Williams is the second Barnsley player to be sent off this season, after Angus MacDonald was dismissed at Sheffield United in August.
It is however Barnsley's first FA Cup sending off since Adie Moses saw red in a Quarter-Final defeat to Tottenham in 1999.
Barnsley have lost their past three matches when having a man sent off.
It is only the second time this season Barnsley have lost when scoring the opening goal (2-1 defeat to Ipswich in August)
Milwall scored with every shot on target, the third time this has happened against Barnsley this season (Derby 0-3, Ipswich 1-2)
Millwall have been drawn at home nine times in their last ten FA Cup ties, and have won seven of those home matches.
This was the first time Millwall had conceded a home FA Cup goal in five against league sides.
Barnsley have scored in three successive matches for the first time since November (run of four)
There were only 11 fouls committed in the game (3 by Millwall, 8 by Barnsley) the lowest amount in a Barnsley game this season.
George Moncur (for Isgrove 64)
Mamadou Thiam (for Hammill 64)
Stevie Mallan (for Potts 73)
Nick Townsend, Jacob Brown, Ben Williams, Will Smith.
FA Cup Third Round results:
Friday 5th January 2018:
Liverpool 2-1 Everton
Milner pen 35 Sigurdsson 67
Van Dijk 84
Manchester United 2-0 Derby County
Saturday 6th January 2018:
Fleetwood Town 0-0 Leciester City
Middlesbrough 2-0 Sunderland
Aston Villa 1-3 Peterborough United
Davis 8 Marriott 75, 90+3, Tafazolli 83
Birmingham City 1-0 Burton Albion
Blackburn Rovers 0-1 Hull City
Bolton Wanderers 1-2 Huddersfield Town
Derik 64 Van La Parra 51, Williams 52
AFC Bournemouth 2-2 Wigan Athletic
Mousset 55 Grigg 4, Hyndman og 29
Brentford 0-1 Notts County
Cardiff City 0-0 Mansfield Town
Carlisle United 0-0 Sheffield Wednesday
Coventry City 2-1 Stoke City
Willis 24 Adam pen 54
Doncaster Rovers 0-1 Rochdale
Exeter City 0-2 West Bromwich Albion
Rondon 2, Rodriguez 25
Fulham 0-1 Southampton
Ipswich Town 0-1 Sheffield United
Manchester City 4-1 Burnley
Aguero 56, 58 Barnes 25
Bernardo Silva 82
Milwall 4-1 BARNSLEY
O'Brien 35,56 Potts 11
Newcastle United 3-1 Luton Town
Perez 30, 36 Hylton 49
Queens Park Rangers 0-1 Milton Keynes Dons
Stevenage 0-0 Reading
Watford 3-0 Bristol City
Wolverhampton Wanderers 0-0 Swansea City
Wycombe Wanderers 1-5 Preston North End
O'Nien 45+1 Harrop 2, 85, Browne 38, pen 78, Horgan 50
Yeovil Town 2-0 Bradford City
Norwich City 0-0 Chelsea
Sunday 7th January 2017:
Newport County 2-1 Leeds United
Shaughnessy og 76 Berardi 9
Shrewsbury Town 0-0 West Ham United
Tottenham Hotspur 3-0 AFC Wimbledon
Nottingham Forest 4-2 Arsenal
Lichaj 20, 44 Mertesacker 23, Welbeck 79
Brereton pen 64
Dowell pen 85
Monday 8th January 2017:
Brighton and Hove Albion 2-1 Crystal Palace
Stephens 25 Sako 69
Sky Bet Championship Matchday Twenty Seven: Barnsley v Wolverhampton Wanderers, Saturday 13th January 2018. Kick-Off: 3pm
On The Road:
Sky Bet Championship Matchday Twenty Eight: Aston Villa v Barnsley: Saturday 20th January 2018. Kick-Off: 3pm