Jabo Ibehre fired home the equaliser Leyton Orient richly deserved 14 minutes from time to earn his side a point.
Cambridge, desperate for points to spark their promotion push, looked as if they were on their way to a smash and grab victory after Tom Youngs forced home a corner from Luke Guttridge ten minutes before half time.
But had the O's taken their early chances they would have been firmly in control of the game before United's out of the blue opener.
Ibehre, John Martin and Matthew Lockwood all had chances to put Orient ahead in the first 30 minutes before Youngs toed the visitors in front.
Despite the setback the home side continued to press and Matthew Brazier went close on the stroke of half time but was denied by the alert Shaun Marshall.
After the break Orient kept up the pressure as Ibehre, Lee Thorpe and Lockwood were all denied by the hugely impressive Marshall in the Cambridge goal.
But Orient's pressure finally told when substitute Chris Tate, who had only been on for two minutes, headed on for Ibehre to smash home the leveller.
Oreient went flat out for the winner and Marshall saved Lockwood's free kick before Orient's claims for a penalty were waved away by referee Ian Williamson after Ibehre was sent to the floor.
Cambridge boss John Taylor admitted after the game: "We had to soak up a lot of pressure and had to battle hard to gain a draw.
"We have picked up a couple of injuries which is bad news, but you can never underestimate the value of an away point." Orient boss Paul Brush said: "I don't want to talk about the referee I'd rather talk about the game - but I did not think a single decision went our way.
"We were unlucky not to win the game and showed a lot of character to keep battling away after we had played so well and then fallen behind.
"We got a point but really could of had all three."