Bangkok United rallied from two goals down to beat leaders Buriram United in Thai League 1 on Tuesday night.
The loss was the Thunder Castle's first of the season and also slammed the brakes on their march to the title.
Buriram would have retained the title if they had beaten the Angels.
However, they remain heavy favourites to lift the trophy as they are nine points ahead of Bangkok United with five games remaining.
Supachai Chaided scored twice in the 11th and 33rd minutes to give visiting Buriram a 2-0 lead, and Rattanakorn Maikami's own goal cut the deficit to 2-1 before half-time.
In the second half, Thitiphan Puangchan made it 2-2 after 66 minutes and Willen Mota gave the home team a 3-2 lead in the 78th minute.
Buriram's Goran Causic equalised for the visitors moments later.
As the match was heading for a draw, Mota shocked the champions with his last-gasp strike.
"We are happy to win the match. Our players had motivation to score against Buriram," said Bangkok coach Totchtawan Sripan.
Buriram coach Masatada Ishii said he took responsibility for the defeat.
He said the team's target remains winning all three major domestic competitions for the second season in a row.
Elsewhere on Tuesday, Port came back from two goals down to beat hosts Chiang Rai United 3-2.
Muang Thong United defeated Lamphun Warriors 3-1, and Sukhothai beat Chonburi 1-0.