Steve Coppell's Reading surprised Manchester United at Old Trafford to earn an FA Cup fifth round replay.
Michael Carrick unleashed a shot from outside the edge of the box that rocketed past Adam Federici into the net to give United a first-half lead.
Cristiano Ronaldo had three shots on goal in the opening stages of the second half but all of them went wide.
And against the run of play Brynjar Gunnarsson powered a header past Tomasz Kuszczak from a corner to make it 1-1.
Gunnarsson had last played on New Year's Day, when he scored in the 6-0 win over West Ham in the Premiership, but he showed no signs of rustiness in the way he took his goal.
The last 10 minutes provided a thrilling finale as Federici brilliantly denied substitute Henrik Larsson with a fingertip save, while Wes Brown had to head clear as Reading threatened.
In stoppage time Louis Saha broke clear but pulled his shot wide.
With both sides resting key players and Reading relying on a counter-attacking strategy, the game had a slightly muted feel about it before Gunnarsson's equaliser.
Beforehand Seol Ki-Hyeon had done his best to provide Reading with an outlet on the right of midfield but all too often Dave Kitson was on his own when the South Korean delivered his crosses.
Ulises de la Cruz had Reading's only real chance in the first half when he almost stabbed the ball past Tomasz Kuszczak.
Park had a number of chances for United in that muted opening 45 minutes but his finishing never quite matched his purposeful running.
Carrick also had an impressive game for United and the England midfielder provided the cross of the game early on.
Andre Bikey failed to cut out Carrick's cross but the ball bounced off Ole Gunnar Solskjaer and Federici made a comfortable save.
Solskjaer had a goal ruled out for offside after Park blocked Bikey's clearance, with the ball ricocheting to the Norwegian and television replays appearing to show the United striker was onside.
Ronaldo set up United's goal, slipping the ball to Carrick, whose shot arrowed through a crowd of players into the corner of the net.
Early in the second half Ronaldo's volley almost crept inside the post after it took a deflection off a Reading player.
Just past the hour mark, Park slipped a superb pass through to Ronaldo but the Portuguese winger's effort went narrowly wide.
And Ronaldo had a third chance to extend United's lead after good work by Solskjaer but once more he failed to hit the target.
Manchester United manager Sir Alex Ferguson:
"When you are leading by just one goal in a cup tie that is nothing.
"It was a good team performance and we deserved to win, but we just couldn't take the chances we had in the game.
"The replay is not a big issue for us. We have a terrific squad and we don't have any backlog of fixtures."
Reading manager Steve Coppell:
"It will more or less the same side for the replay.
"We won't change from what we have been doing this season in the cups.
"It is disrespectful to question their ranking - we had five internationals, last season's top Championship scorer and a potential Australian goalkeeper in this team."
Man Utd: Kuszczak, Brown, Vidic, Silvestre, Heinze (Evra 73), Ronaldo, Fletcher, Carrick, Park (Scholes 73), Saha, Solskjaer (Larsson 73).
Subs Not Used: Heaton, O'Shea.
Goals: Carrick 45.
Reading: Federici, De la Cruz, Bikey, Ingimarsson, Shorey, Ki-Hyeon (Little 89), Oster (Sodje 86), Gunnarsson, Sidwell, Convey (Hunt 71), Kitson.
Subs Not Used: Hahnemann, Lita.
Booked: Ki-Hyeon, Bikey. Goals: Gunnarsson 67. Att: 70,608 Ref: G Poll (Hertfordshire).