Jon Otsemobor scored a last-gasp winner to send MK Dons into the FA Cup third round following a 2-1 triumph over bitter rivals AFC Wimbledon.
Jack Midson's diving header had looked set to earn Wimbledon a replay after cancelling out Stephen Gleeson's stunning opener, but Otsemobor flicked home in the last minute to break the visitors' hearts.
The match was the first between the two clubs since 1988 FA Cup winners Wimbledon FC were uprooted from south west London in 2003 and moved over 50 miles north of the capital to the Buckinghamshire town, before being renamed MK Dons the following year.
Angelo Balanta came close when he curled the ball just wide of the post after a neat turn on the edge of the box before Gleeson broke the deadlock on the stroke of half-time, blasting a 30-yard shot into the top-right corner.
Midson made the most of a rare Wimbledon counter-attack to equalise after 59 minutes, glancing a fine diving header past David Martin from Toby Ajaya's cross. A number of Wimbledon fans spilled onto the pitch following the goal, but stewards quickly restored order.
In a frantic finale, Ryan Lowe had a goal ruled out for offside for Milton Keynes and Wimbledon skipper Steven Gregory was denied by Martin before Otsemobor hung out a leg to backheel substitute Zeli Ismail's mis-hit shot over Sullivan to spark wild scenes of celebration for the hosts.
Meanwhile, David Mooney and Dean Cox proved the heroes again for Leyton Orient as their doubles killed off Alfreton in a thrilling 4-2 victory. Blue Square Bet Premier side Alfreton took the lead early on and laid siege to Orient's goal having pulled it back to 3-2 but the npower League One side clung on.
The hosts flew out of the blocks and were ahead on four minutes when top-scorer Paul Clayton fired under Ryan Allsop from 12 yards. Ben Tomlinson then struck a post and Josh Law's deflected free-kick hit Allsop's bar as the non-league side dominated but Dean Cox levelled for Orient.
And on the half-hour mark the visitors were ahead when Lee Cook's corner was flicked on and Mooney volleyed home. Alfreton were shell-shocked and Mooney had his second just six minutes later, heading home Cook's left-wing cross.
On 52 minutes Alfreton pulled a goal back as Tomlinson scrambled home from close range and while the hosts peppered Allsop's goal - Law hitting the bar with another free-kick and Tomlinson from open play - Cox grabbed another late on.