Perth Glory players were fuming after a stoppage-time equaliser was disallowed in their 2-1 loss to Sydney FC at nib Stadium.
Former Glory player Danny De Silva gave Sydney a 2-0 lead in the 87th minute, adding to Adam Le Fondre's first-half strike.
But in a frantic finish on Friday night, Chris Ikonomidis pulled a goal back in the 90th minute before Brendon Santalab thought he had equalised in the third minute of stoppage time.
However, the goal was disallowed for Joel Chianese's foul on goalkeeper Andrew Redmayne. Replays showed Chianese made the slightest of tugs on Redmayne's arm.
Glory players were furious about the decision, with the result ending their unbeaten start to the season, while the 10,356-strong crowd also voiced their displeasure.
The defeat means Glory will drop to second if Melbourne Victory beat Melbourne City on Saturday.
Sydney are now just three points adrift of Glory, with the two teams to meet again in Perth next month.
Glory's hopes of notching a third straight clean sheet were dashed after just 18 minutes when they failed to clear a corner.
Aaron Calver made the most of the scrap to tap the ball in the direction of Le Fondre, who slotted home his seventh goal of the season.
Glory striker Andy Keogh came close to scoring in the 31st minute, only for his strike to be blocked by Redmayne.
De Silva, a Glory crowd favourite when he made his debut at the age of 15, came back to haunt his former side.
The speedy attacker sneaked past Glory's defensive line, brushed past Dino Djulbic and slotted home Sydney's second goal.
Ikonomidis ignited the crowd when he pulled a goal back three minutes later, before Santalab was robbed of his first goal of the season by the referee's injury-time decision.