Glentoran striker Curtis Allen scored with the last kick of the game to earn a 1-1 draw away to Linfield.
The two dropped points mean Linfield are replaced as Premiership leaders by champions Crusaders on goal difference.
It had looked like Linfield were going to win after midfielder David Kee shot them into a 63rd-minute lead.
But Glentoran did not give up and after a corner deep into stoppage-time the ball fell to former Linfield player Allen in the box and he hooked in.
Early in the second half, with the score at 0-0, Glentoran's Allen had a goal disallowed because Fra McCaffrey, who headed the ball down, was offside.
Linfield's Andrew Waterworth also had an effort ruled out by an offside flag.
Late in the game, scorer Kee was replaced by substitute Jamie Mulgrew who came on to an ovation from the home fans to acknowledge his 400th appearance for the Blues.
It was Linfield's first match since the resignation of team boss Warren Feeney, the former Northern Ireland striker who left to join Newport County as assistant manager.
Linfield have lost just one of their 11 Premiership fixtures this season, but their second successive draw saw them surrender the leadership as second-placed Crusaders beat bottom club Warrenpoint Town 1-0 at Seaview.
What they said
Glentoran scorer Curtis Allen: "A point was the least we deserved because we dominated most of the game.
"To score the late goal was great - it was just a pity it was not the winner.
"We have been poor recently, but today we showed good character and the tactics from the coaching staff were spot on.
"We have dominated games and then conceded, and it happened again today, but we fought back and showed we are all in this together."
Linfield assistant manager Andy Todd: "It was a tight game, a typical derby, and to concede so late in the game was disappointing.
"But the warning signs were there with a long throw just before that.
"We have to be alive to those situations. It is something we have to address, because it will cost us in the long run."