Experienced striker Kevin Lisbie levelled for 10-man Leyton Orient in the sixth minute of injury-time against Sheffield United.
The hosts had Charlie Daniels sent off for a rash challenge on Lee Williamson, who had to be taken off.
Chris Porter put the Blades ahead after Ched Evans and Kevin McDonald saw their efforts blocked.
The O's equalised when visiting keeper Steve Simonsen punched Dean Cox's free-kick to Lisbie, who lashed home.
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Leyton Orient boss Russell Slade told BBC London 94.9:
"I can't comment [on the refereeing decisions] otherwise it would be a very long interview.
"But there are about 20 decisions that we're not too happy about - everyone in the ground knew why we were 1-0 down."
Sheffield United boss Danny Wilson told BBC Radio Sheffield:
"Well I have to say it was a scrappy game in all honesty.
"I think we had the clearest cut chances in the first half in particular and it was just a scrap, we had to make sure we got something out of the game.
"It was very frustrating because we've lost two points. Some of the decisions we made in the last few minutes were what caused the goal."
Leyton Orient 12 Butcher 05 Cuthbert 06 Forbes 25 T Clarke (Laird - 46' ) 07 Cox 08 Dawson Booked 11 Daniels Dismissed 14 McSweeney Booked 16 Spring (Porter - 86' ) 09 Lisbie Booked 39 Mooney (Omozusi - 75' ) Substitutes 24 Lovelock 02 Omozusi 15 Porter 17 Laird 10 Cureton Sheffield Utd 01 Simonsen 02 Lowton 15 Collins 19 Maguire Booked 30 Williams 04 Montgomery Booked 27 McDonald 28 Quinn 32 M Phillips (Williamson - 70' ) 21 Porter 26 Clarke (Evans - 57' ) Substitutes 25 Long 03 Lescinel 08 Doyle 14 Williamson (Doyle - 76' ) 09 Evans