MK Dons' road to Wembley ended abruptly at the first hurdle on Saturday as they threw away a first half lead to lose 3-1 to Grimsby Town.
Kieran Agard's seventh goal of the season after 23 minutes was the lone piece of action from the first half, but a renewed Grimsby emerged for the second and booked their spot in the next round with goals from Elliot Embleton, Wes Thomas and Charles Vernam.
It is already the fourth time Dons have conceded three in a game this season, all of which have come in cup competitions.
Paul Tisdale, fresh from being named League 2 Manager of the Month for October, fielded the same side which ran Crawley Town ragged a week earlier for their trip to Blundell Park.
After a slow opening 20 minutes, Dons took the lead through Agard, but the goal was all about Callum Brittain's persistence. Unwilling to let a ball roll out of play, Brittain put defender Akin Famewo under pressure and pinched the ball from him on the by-line, pulling it back for Agard to routinely wrap home his seventh goal of the season.
The home side rallied when Embleton twice tested keeper Lee Nicholls from the edge of the box, but neither effort really had him worried.
At the other end, Baily Cargill twice sent attempts over the bar, while Agard could have made it 2-0 just a minute before the break when he poked the ball around keeper James McKeown, but could not get on the end of it as it rolled just wide of the post.
It did not take long for Grimsby to restore parity in the second half though, and it came courtesy of a wonder-strike from Embleton. Making space for himself on the edge of the box five minutes after the restart, the midfielder unleashed an unstoppable effort into the top corner, leaving Nicholls no chance.
The keeper will feel he should have done better 10 minutes later though when Wes Thomas poked home from inside six yards after Dons failed to clear what should have been a routine ball into the box, and allowed Grimsby a second bite of the cherry, Thomas finishing with aplomb.
Dons were almost level immediately though, but Rhys Healey's goal-bound effort was headed off the line by Danny Collins.
The home side wrapped it up with 17 minutes to go when Dons once again failed to clear a routine ball into the box, and afforded Charles Vernam the opportunity to find the top corner for 3-1.
The visitors pressed for a lifeline, but Brittain, Ouss Cisse nor Alex Gilbey were able to find the target, while Robbie Simpson could only tamely head into the arms of McKeown.
Referee: Carl Boyeson
Attendance: 1,991 (154)
Grimsby Town: McKeown, Collins, Hessenthaler, Fameow, Clifton, Vernam (Welsh 77), Embleton (Woolford 90), Davis, Hendrie, Pringle, Thomas (Cardwell 87)
Subs not used: Hall-Johnson, Cook, Cardwell, Fox, Russell
MK Dons: Nicholls, Cargill, Walsh, Williams, Brittain, Lewington, Houghton, Gilbey, Agard, Healey (Simpson 73), Aneke (D'Ath 64)
Subs not used: Moore, Cisse, Watson, Hancox, Simpson, Jackson
Booked: Houghton, Williams, Walsh, Welsh