When Iliman Ndiaye calmly slotted the ball past Orjan Nyland with four seconds of the 90 minutes remaining it looked like that sequence would continue and the Blades had salvaged at least a point from a performance that deserved nothing.
Even the Bramall Lane faithful suddenly believed, propelled from their seats by a rare moment of attacking incision, the momentum to go and pinch victory was now theirs.
Ninety seconds later and the air was sucked back out of the majority of the 29,145 in attendance when Tom McIntrye poked the ball past Wes Foderingham from Michael Morrison’s knock down.
Heckingbottom’s home stand was over, and more pertinently, a golden opportunity to climb up to fifth at least and take a step closer to securing a play-off spot and maintain the dream of a Premier League return had been spurned.
A big play had been made of the fact United had the bulk of their remaining league games at home (six of the last nine), seemingly an advantage over their fellow play-off contenders given the record under Heckingbottom on home soil.
But the goals had been drying up, one against QPR, none against Bournemouth and now barely a shot on target until the dying embers when they were chasing the game.
Billy Sharp was the obvious elephant in the room – his absence since hobbling off against Barnsley at the start of this run-in never more keenly felt than here against Reading. Not so much for his goals, but his running off the ball, his darting into channels, his constant harrying of defenders.
Oliver McBurnie could not match that work-rate, that holding up of play and that linking of midfield and attack.
Such guile was missing from a largely ponderous performance from the Blades, given what was at stake.
They were hesitant going forward against Reading’s 4-5-1 formation, and seemed to struggle picking out Reading’s runners from deep. John Egan was strong at the back but Ben Davies alongside him struggled.
Lucas Joao, the former Sheffield Wednesday man, tormented them with his direct running, and blasted Reading into the lead on 17 minutes when Josh Laurent played him in behind the home defence.
Joao took a touch and rifled the ball past Foderingham before gleefully celebrating in front of the Kop.
Having made his name across the city at Hillsborough, it was a particularly pleasing moment for the Portuguese.
He should have had a second though moments later after Filip Uremovic needlessly gave the ball away trying to clear his lines. Reading pounced, the ball was worked to Joao who jinked one way and then the other before guiding his shot against the outside of the post.
When United did attack, George Baldock venturing forward from the right was the recipient of their best two chances but on each occasion his first touch betrayed him.
Sander Berge replaced Uremovic at half-time and the graceful Norwegian immediately gave United more thrust from midfield but still they could barely muster any chances of note.
Too often when a team-mate was on the ball, too many United players stood ball-watching instead of creating space.
It was the same at the back, hesitant, waiting for someone else to commit. Only Egan emerged with any real credit.
The Republic of Ireland defender almost stole the show up front.
First he had a golden opportunity to equalise on 89 minutes when Oliver Norwood picked him out with a sumptuous cross, but he headed straight at Nyland. At least he’d hit the target.
But then came his most telling contribution, a deft overhead to retain possession as he moved into the penalty area and then with a sight of goal or a simple pass out wide to Ndiaye he chose the right option, a lovely weighted pass straight into the 22-year-old Frenchman’s path and he did the rest.
Only one winner from there, surely. But this is the Championship.
Free-kick pumped into the box, Morrison got the better of Davies and McIntyre stabbed home.
Reading deserved the win, their manager Paul Ince confessing Heckingbottom told him as much after that breathless finale.
Heckingbottom’s home run is over, his team undone by Reading’s rearguard and the pace with which they broke. United can be better, much better than this, and they’ll need to be if they are to book a play-off place.
Sheffield United: Foderingham, Uremovic (Berge 46), Egan, B Davies, Baldock, Norwood, Hourihane (Ndiaye 70), Stevens, Gibbs-White, Osborn, McBurnie (Jebbison 55). Unused substitutes: A Davies, Basham, Berge, Robinson, Norrington-Davies.
Reading: Nyland, Morrison, McIntyre, Holmes, Yiadom, Drinkwater, Laurent, Baba, Ejaria, Ince (Dele-Bashiru 88), Joao (Azeez 77). Unused substitutes: Thomas, Southwood, Hoilett, Barker, Ehibhationham.
Referee: G Ward (Surrey).