Ballymena blew a two-goal lead and missed a penalty before Glentoran stormed back to snatch victory with two stoppage-time goals from Colin Nixon.
Nathan McConnell shot United into the lead in the 32nd minute and they had a chance to go two up but keeper James Taylor saved Andy Smith's spot-kick.
McConnell netted his second after a quick Ballymena break on 72 minutes.
Andrew Waterworth stabbed in for the Glens in the 83rd minute before Nixon's sensational late double.
In the first minute of stoppage-time, Nixon headed in from Kyle Neill's cross.
Then in the 94th minute Nixon snatched the winner after Jamie McGovern's crossfield ball had been nodded on by Gary Hamilton.
The Sky Blues will look back at Smith's 23rd-minute penalty miss as a key moment of an incident-packed match.
The penalty was awarded when Nixon brought Ormon Okunaiya down.
Smith attempted a casual dummy but keeper Taylor guessed right and dived to save.
Still, when McConnell played a one-two with Smith before shooting in Ballymena's second it looked like the champions were going to fall to defeat.
But, just as they did against Cliftonville on Saturday, they staged a late fightback to take all three points.
Waterworth got the ball rolling, netting after Hamilton's header had come off a post from Neill's free-kick.
Ballymena still had their noses in front going into stoppage-time but Nixon's double turned the match on its head.
The win puts the champions level on points at the top of the table with Belfast rivals Linfield and Crusaders.
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