Rushden & Diamonds looked to be cruising to their second away win of the season after Alex Hay and Craig Dove either side of the break put them firmly in control.
But 17-year-old Orient substitute Efe Echanomi cut the deficit with a header on 88 minutes then Daniel Chillingworth netted a bizzare leveller in stoppage time.
It was rough justice on Diamonds as they caused Orient plenty of problems early on.
Hay was denied by home goalkeeper Lee Harrison and then Jason Taylor almost toe-poked the visitors ahead.
Taylor saw another effort saved by Harrison as Rushden looked dangerous on the break.
Orient rallied and Lee Barnard was denied by visiting goalkeeper Lee Worgan after an excellent through ball from the immaculate Michael Simpson.
But Rushden deservedly broke the deadlock just before the half hour when Harrison missed his clearance and Hay accepted the gift-wrapped opportunity to roll the ball into an empty net.
Orient hit back and almost levelled as Chillingworth was denied by Worgan before Andy Scott fired inches wide.
Orient stepped up the pressure in the second half and they forced a total of 19 corners throughout the match, but it was Rushden who grabbed a second goal against the run of play when Dove netted superbly with a 25-yard chip into the top corner.
With Rushden defending doggedly they were reduced to 10 men 12 minutes from time when Sean Connolly saw red for a second bookable offence.
Orient threw caution to the wind but Rushden looked to have weathered the storm until two minutes from time when substitute Daryl McMahon crossed for Echanomi to head home from close range.
Then, with 90 minutes up, Chillingworth's hopeful punt forward looked like going over the bar until it swirled in the wind and bounced into the net.