Michael Walther’s Barkingside looked set for a crucial win as they led through Ryan Imbert’s first-half strike, but Calvin Poku popped up with a late double to down his former club.
Ricky Eaton and May's league leaders started well. First Reside Coxi-Sebastfao saw a shot deflected over for the hosts, before Ryan Karrer did well to save a shot at his near post.
After withstanding more Redbridge attacks, 'Side soon had their first effort, but Kofi Billzy shot over after a good run. Redbridge then wasted a glorious chance to take the lead when Eljay Worrell's cross was somehow headed over by Brian Moses.
'Side made Moses pay for that miss by taking the lead shortly after. Imbert brilliantly bursting into the area on 30 minutes to score past Lewis Greene. It was the forward's third goal since rejoining the club on-loan from local rivals Romford in December.
Both goalies were in action in a thrilling end to the half. Karrer again keeping out Moses before saving well from Remi Nelson. Down the other end 'Side almost doubled the lead but Greene reacted well to deny Billzy.
Karrer was having a great game in goal and after saving a deflected Coxi-Sebastfao effort, produced one of the saves of the season next. The keeper produced a remarkable save from Sipho Mtyanda's free-kick, tipping a fine effort onto the bar. It ensured the visitors went into half-time ahead.
Redbridge made a bright start to the second half, with forward Moses continuing to look bright as Worrell saw a shot blocked. Imbert then tested Greene in goal before Karrer was forced into another save from Coxu-Sebastfao.
'Side then had a chance on the break, but Greene saved well from Garrett Kelly following good play by Mattheus Santos.
With time running out Poku, who enjoyed a great spell for 'Side under Matt Frew, came on for the hosts. The forward's impact was to change the game.
After Jayden Mitchell-Brown was sent off on 86 minutes for a second-booking, the game well and truly turned. From the resulting Shaun Harris free-kick, Moses knocked the ball down for Poku to finish and equalise on 88 minutes.
If that wasn't hard enough on 'Side, in the first minute of injury-time Moses headed in for Poku to bag a winner.
There was still time for fellow substitute Jacob Dingli to hit the post, as the hosts went close to grabbing a third.
'Side will look to bounce back from this disappoint when they face Waltham Forest away on Wednesday night. Kick-off at Wadham Lodge is at 7:45pm.
'Side: Ryan Karrer, Akil Clarke (Bradley Joseph), Aaron Hunwicks, David Etcheri, Joseph Huston-Mendy, Michael Bottone, Roddy Lemba, Kofi Billzy, Ryan Imbert (Jayden Mitchell-Brown) Garrett Kelly, Mattheus Santos (Ricardo Bailey)
Subs Not Used: James Briggs, Ben Clarke