Geoff Horsfield climbed off the bench to strike the extra-time winner and see Sheffield United past a plucky MK Dons side into the Carling Cup third round.
Drewe Broughton rammed home his first Dons goal for the opener, but Chris Lucketti levelled with a neat header.
Nicky Law then pounced on a rebound to put the Championship side ahead but Jon-Paul McGovern's deflected shot took the League One Dons into extra-time.
But Horsfield slotted the winner from close range to break the hosts' hearts.
Milton Keynes Dons: Abbey, Diallo, Murphy, Swailes, Stirling, Edds (Wilbraham 86), Navarro, Carayol (Dyer 62), McGovern, Gallen (Wright 68), Broughton.
Subs Not Used: Gueret, Kelly.
Goals: Broughton 12, McGovern 78. Sheff Utd: Kenny, Geary, Morgan, Lucketti, Kilgallon, Law (Horsfield 79), Alan Quinn, Leigertwood, Carney (Tonge 91), Stead, Sharp (Shelton 68).
Subs Not Used: Bennett, Ashmore.
Booked: Geary, Stead, Shelton.
Goals: Lucketti 20, Law 55, Horsfield 104.
Ref: Phil Joslin (Nottinghamshire).