East Bengal's title aspirations were dealt a body blow as Gokulam Kerala FC stunned them with a come-from-behind 2-1 win at the EMS Corporation Stadium.
Yusa Katsumi (45+4 minutes) opened the scoring from the penalty spot for the away side but Kivi Zhimomi (51st) restored parity before an own goal by substitute Salam Ranjan Singh (87th) made matters worse for the Kolkata heavyweights.
Both captains, Arnab Mondal of East Bengal and Mohammed Irshad of Gokulam, were sent off in second half added time.
Mondal picked up a second yellow for his lunge on Henry Kisseka while Irshad was given the marching orders for putting his arm on the face of East Bengal's Lalramchullova.
The result saw East Bengal remain third in the table with 26 points from 15 fixtures, three points adrift of second-placed Minerva Punjab FC (29 points from 14 games) and five of league leaders Neroca FC who have 31 from 16 matches.
Meanwhile, Gokulam moved to the eighth spot with 16 points from 14 games.
There were no surprises in the starting line-up for the hosts as coach Bino George opted for the same side that beat Mohun Bagan five days ago.
For the visitors, coach Khalid Jamil made one change from their last game against Minerva with Samad Ali Mallick replacing Salam Ranjan Singh.
After almost a goalless first half where chances were few and far between, East Bengal took the lead when Cavin Lobo was brought down inside the box by Nigerian defender Emmanuel Chigozie.
Katsumi stepped up to take the penalty and made no mistake to net his sixth of the campaign and go into the break 1-0 up.
The visitors' joy was shortlived as a poor piece of goalkeeping cost them dear. After a series of cohesive passes, Henry Kisekka released Zhimomi on the edge of the box with the latter shooting on goal.
Gokulam had two more chances to take the lead but on both occasions Kisseka was denied by the woodwork.
Finally, justice prevailed as the home team shocked their more fancied opponents to nose ahead for the first time in the game.
Arjun Jayaraj crossed the ball inside the box for Kisseka after another neat build-up play by the lowly outfit. But instead, Salam Ranjan deflected it into his own net.
In the final minutes of the game, after East Bengal were trailing 1-2, a fracas broke out between players of the two teams.
Mondal picked up his first booking during the heated exchanges as he pushed Sushant Mathew on to the floor. Gokulam's Irshad, meanwhile, was sent off.
East Bengal next take on Chennai City FC at home on February 24 while upbeat Gokulam will travel to Punjab to play Minerva on Tuesday.