United boss Sir Alex Ferguson and his Reading counterpart, Steve Coppell, made six and seven changes respectively to their sides.
From the way a scrappy first half developed, it looked as if either team would struggle to produce a goal of any description in this fifth round FA Cup tie at Old Trafford.
But deep into first-half stoppage time, Michael Carrick gave United the lead with a super low shot.
It looked as if that might prove to be the winner until United went to sleep midway through the second half enabling Brynjar Gunnarsson to capitalise on poor marking at a John Oster corner to head home the equaliser.
The first half had produced little in the way of chances, although what were created were mainly by United.
In the fifth minute, superb work by Louis Saha on the right corner of the Reading box set up Ji-Sung Park who blasted the ball over the bar from ten yards.
Only a fine intervention by Andre Bikey prevented Ole Gunnar Solskjaer from firing United ahead soon after.
Saha showed super skill just before the half-hour when his run from the centre circle took out three Reading players before the Frenchman shot over from 12 yards.
Park missed another good effort seconds later when he fired over from 12 yards after being set up by Wes Brown.
United should have taken the lead in the 34th minute. Park charged down a clearance from Bikey and the ball fell kindly to Solskjaer ten yards from goal and, although the Norwegian slotted his shot home, he turned around in astonishment to see the effort ruled out for offside.
Reading's best chance of the half came in the 40th minute when Ulises de la Cruz shot into the side-netting from eight yards after a fine one-two down the right with Oster.
But, with the first half moving into the second minute of stoppage time, United broke the deadlock.
Brown found Cristiano Ronaldo in the middle of the Reading half and his clever turn and flick set up Carrick to score with a superb low 25-yard right-foot shot which beat Reading goalkeeper, Adam Federici, before nestling in the bottom corner of the net.
United should have sealed the result in the 63rd minute. Park's clever chip into the Reading box found Ronaldo racing clear of Gunnarsson, but, with only Federici to beat, he slipped his shot wide of the goalkeeper and inches beyond the far post from eight yards.
Ronaldo went close with a 20-yard shot soon after, but United fell for the sucker-punch in the 67th minute.
Oster's deep corner to the back of the penalty area caught the United defence napping and Gunnarsson powered his way forward to score with a bullet header from 12 yards.
Ferguson brought on Paul Scholes, Henrik Larsson and Patrice Evra in an all-out bid to find a winner.
In a grandstand finale, Federici brilliantly tipped behind Larsson's close-range shot while Saha raced clear from just inside the Reading half before hitting his shot narrowly wide from the edge of the box.
But Reading almost grabbed a winner themselves late on when Ki-Hyeon Seol's low cross from the right evaded United goalkeeper, Tomasz Kuszczak, and Nicky Shorey got in a shot at the far post which had to be turned behind by Brown.