Ola John came off the bench to rescue a point for the Royals when it seemed Chris Powell's men would hold on for all three at Madejski Stadium, following first half goals from Jamie Paterson and Nahki Wells.
Reading had come from behind once already, Oliver Norwood's free kick foxing Jed Steer at his near post to bring it back to 1-1. But once Wells had restored the visitors' advantage, it took some resolute defending from the Terriers to hold the hosts at bay until John squeezed home the leveller with five minutes remaining.
Royals boss Steve Clarke made three changes to the side that held the league leaders last weekend, bringing in a fit again Anton Ferdinand, Lucas Piazon and Jordan Obita for Michael Hector, Andrew Taylor and Orlando Sa.
But the Royals got off to the worst possible start, conceding after little more than a minute. Nahki Wells intercepted an Obita pass and threaded the ball into the feet of Jamie Paterson
who calmly slid the ball wide of Ali Al Habsi for the opener.
It took a good save from the Omani stopper to deny Town a second moments later after Harry Bunn had marauded into the Royals box and fed the ball into Paterson's feet - this time Al Habsi saved well low to his left and smothered the ball before it could roll behind for a corner.
With nearly a quarter of an hour gone, Obita came close to levelling the scores. Found by Williams on the left, the full-back cut inside and scurried between two defenders and into the area, before firing right-footed at goal. It deflected off a Huddersfield leg and spun inches wide of the far post with McCleary unable to turn it in.
Soon it was 1-1 though, the set piece wizardry of Oliver Norwood
restoring parity. Standing over a free kick on the left side, he went for goal when everyone was expecting a cross, fooling Jed Steer who could do nothing to keep it out at his near post.
Reading had found their feet and a good cross found McCleary in acres of space inside the area but as he widefooted just wide, the offside flag was raised on the right side against the Royals winger.
However, against the run of play, the Terriers retook the lead when Nahki Wells
sprung the offside trap to race onto a Bunn pass and sweep the ball past Al Habsi and into the back of the net with an emphatic finish. Complaints to the assistant referee fell on deaf ears and Town led once more.
Obita burst down the left, trying to unlock the visitors defence again and he did well to float a cross in towards the head of Blackman, but the Royals' top scorer couldn't meet it with a firm enough header and the chance fizzled out.
Reading's final ball was missing until a lovely backheel from Blackman gave the ball to Williams 25 yards out and, with the space to shoot, he connected well with a right-footed effort, but Steer got his body behind it to save comfortably on the stroke of half-time.
There was still time for Steer to be tested once more, Obita hanging a cross up for Blackman to get his head to, but the Town keeper held it high on his goalline as the linesman signalled two added minutes at the end of the first period.
Half-time: Reading 1 Huddersfield Town 2
The Royals boss made two changes at the break, bringing Sa and Fernandez on for Piazon and McCleary, while Chris Powell also replaced Jamie Paterson with Joe Lolley.
And within a few minutes of the restart, Reading came within a whisker of an equaliser. Vydra scurried into the box and was tackled, but the ball fell loose to Blackman who hooked the ball goalwards. Steer did enough to parry it against his right post and an offside Sa put the follow-up over.
Ferdinand looped a header over the bar as the home side pressed for a route back into the game, then Sa hurried a shot from the edge of the area, slicing it wide of the target as he was closed down.
Williams was found by Vydra on the break, but his effort was blocked on the hour mark as Reading continued to look for a way through a now-packed Huddersfield defence.
Town were offering very little at the other end, although when Al Habsi failed to cleanly gather a cross from the right under pressure from Lynch, Hudson thankfully hacked over from six yards out.
Frustrations boiled over when Lynch brought down Vydra as he looked for a one-two with Sa, Blackman was booked alongside Bunn too after a small melee in the middle.
Substitute Ola John earned a corner with a deflected drive, then Fernandez drove an effort just over the bar from fully 25 yards out. But time was beginning to run out on the Royals.
Then, with six minutes left on the clock, John found a route to goal. Huddersfield failed to clear a cross from the right and the ball fell to the feet of John
, who weaved his way past a couple of defenders to squeeze home a thunderous low left-footed drive from a narrow angle.
Suddenly, Reading were looking for the win. Fernandez's powerful drive was blocked inside the box, Gunter's pull back almost picked out Sa in the middle, then Blackman rifled wide.
John tested Steer's handling with a low effort before five minutes of injury time were signalled. Norwood fizzed an effort just wide, but Steve Clarke's side couldn't find the last gasp winner the home fans were hoping for and 2-2 it finished.
Full-time: Reading 2 Huddersfield Town 2Reading:
Al-Habsi, Gunter, Obita, McShane (c), Ferdinand, Norwood, Williams (John 74), Piazon (Fernandez HT), McCleary (Sa HT), Vydra, Blackman.
Subs: Bond, Taylor, Cooper, Hurtado.
Goal: Norwood 15, John 85
: Steer, Davidson, Hudson (c), Cranie, Huws, Bunn, Dempsey (Smith 90), Billing, Lynch, Paterson (Lolley HT), Wells (Vaughan 79).
Subs: Murphy, Smith, Lolley, Bojaj, Wallace, Carayol.
Goal: Paterson 2, Wells 26
Yellow: Dempsey, Lynch, Bunn.
Attendance: 14,379 (266 away)
Referee: Mr B Malone