Whilst Arsenal made history in 2003/04 by going an entire English top-flight season undefeated for the first time since Preston in 1888/89, they did suffer defeat in other competitions, with one team in particular getting the better of them twice.
The team in question? None other than Middlesbrough.
With Arsenal cruising to title success back in January 2004, attention turned to the Carling Cup semi-finals, with the 'Gunners' drawn against the Teessiders.
31,000 fans turned out for the first leg at Highbury to watch as Boro shocked the hosts, with Juninho's low drive across Graham Stack in the 53rd minute enough to give his side a slender 1-0 victory going into the second leg.
The defeat was Arsenal's first domestic loss in 30 games, despite the 'Gunners' fielding the likes of Martin Keown, Ray Parlour, Kolo Toure, Edu, Gilberto Silva and Nwankwo Kanu in the starting linuep.
A fortnight later and Arsenal tasted defeat again domestically as they went down 2-1 at the Riverside.
Bolo Zenden opened the scoring for the hosts, dinking the ball over Stack, before Edu quickly responded when he was first to react to nod beyond Mark Schwarzer after Patrick Vieira headed against the bar from a corner:
All the headlines the next day were about young Spaniard Jose Antonio Reyes, who was handed his full debut by Wenger in the game, but contrived put through his own net late on as he attempt to track back and challenge Stuart Parnaby.
If there was one small consolation for the 'Gunners', it was that Boro went on to win the trophy that year, defeating Bolton Wanderers 2-1 in the final at the Milennium Stadium in Cardiff.