Gary Shanahan's injury time winner secured Galway United their first win of the season to leave Dundalk's hopes of a fourth successive league title teetering on the brink.
The 24-year-old, who enjoyed a miserable season at Oriel Park in 2012, popped up in the 93rd minute of play to steer a deflected effort past Gary Rogers to send Eamonn Deacy Park into raptures.
The result leaves the Lilywhites 12 points behind Cork City after the first series of games. Even at this early stage, that looks like an insurmountable gap for Stephen Kenny's side to overcome.
The first 20 minutes drifted by with both goalkeepers barely called into action. Dundalk bossed possession, but a bobbly pitch made things difficult for the players.
The only real chance of note arrived after 15 minutes when Dane Massey's ball was headed dangerously across the face of goal by Michael Duffy but Stephen Folan was on hand to clear.
The visitors looked like they were beginning to take a grip on things and Galway goalkeeper Conor Winn back peddled to gather a Dane Massey cross in the 24th minute.
However, it was Galway who drew first blood two minutes later when Ronan Murray's cross from the left found the head of another ex-Dundalk player, Vinny Faherty, and he guided a header past Gary Rogers for the second year in succession.
Dundalk took time to regroup but they went within inches of making it 1-1 in the 31st minute when Jamie McGrath collected a pass from Stephen O'Donnell only to see his low drive creep wide of the upright.
Stephen Kenny's side did restore parity five minutes later. Brian Gartland's ball into the right channel found McMillan in space and he composed himself, picked his spot and buried the ball into the far corner.
Dundalk had a pep in their step at this stage and Winn was forced to make a fine stop to turn McGrath's vicious half-volley over the top before Patrick McEleney's cross from the left just evaded both McMillan and Chris Shields.
McGrath was involved again in the early stages of the second half, fluffing a good chance to put Dundalk in front after a Michael Duffy cross had bobbled into his path.
Things became very scrappy again after that with both defences well on top. Dundalk winger Duffy did accelerate away from Colm Horgan on the right in the 76th minute but there was nobody in the six-yard box to meet his pull back.
Kenny introduced Ciaran Kilduff and Tommy Stewart in a bid to win the points and the champions had what looked like a good shout for a penalty waved away when Shields went down just inside the area. Referee Ben Connolly, however, was having none of it.
Another substitute, John Mountney and defender Sean Hoare both fired wide with long range efforts in the dying stages and it looked like the game would peter out for a draw but Shanahan's last gasp winner left Rogers helpless and the Dundalk players on their knees.
Galway United: Conor Winn; Colm Horgan, Stephen Folan, Lee Grace, Marc Ludden; Gary Shanahan, Gavan Holohan (Paul Sinnott 90), David Cawley, Kevin Devaney; Ronan Murray (Jesse Devers 83), Vinny Faherty (Padraic Cunningham 86).
Dundalk: Gary Rogers; Niclas Vemmelund (John Mountney 73), Brian Gartland, Sean Hoare, Dane Massey; Chris Shields, Stephen O'Donnell; Michael Duffy, Patrick McEleney, Jamie McGrath (Ciaran Kilduff 79); David McMillan (Thomas Stewart 84).
Referee: Ben Connolly.