Reading manager Nigel Adkins was left to rue a poor start to the second half after a 2-1 defeat by Norwich almost confirmed their relegation.
The Royals were undone by two goals in two minutes at Carrow Road and, despite pulling a goal back late in the game, they were unable to recover.
Adkins' side could now be consigned to the Championship before they play QPR next Sunday if results go against them.
He told Sky Sports: "I thought we did well first half but it was disappointing how we started the second half.
"We said it was important how we dealt with the balls they play into the box and the physicality and we did it well for the most part.
"But for that 15-minute spell we just couldn't deal with them - that led to the two goals."
But, despite their predicament, Adkins vowed there would be no let-up in the work rate of his players.
He continued: "At the end of the day we've lost a game of football. We're in a poor position in the league. We can look at it and take positives from it but we've lost.
"We carry on. The players worked hard and showed a lot of endeavour and we'll keep working hard.
"We'll analyse this game, learn the lessons from it and get ready for the next game."
Agard applied the deft close-range finishing touch after skipper Johnny Mullins headed down Joe Skarz's free-kick into the six yard box.
Lee Frecklington missed a gilt-edged opportunity to fire United just before half time when he was denied by a superb Jake Cole save.
Ben Pringle's angled 45th minute header across the area found Frecklington on the right but his shot was brilliantly turned around the post by Argyle's keeper.
Argyle's best first half chance came early on as Andres Gurrieri's third minute corner caused a goalmouth scramble, eventually cleared by Mullins.
Rotherham started the second half brightly with striker Alex Revell going close in the 47th minute and Agard heading wide from Pringle's cross minutes later.
Argyle responded with striker Reuben Reid flashing a stooping header inches wide from Onismor Bhasera's pinpoint near-post cross from the left in the 53rd minute
Reid should have scored from Bhasera's next cross, a minute later, but fired high and over from 12 yards.
The home side dominated much of the first half as Jamille Matt, Gareth Evans and Ryan Crowther all enjoyed chances to open the scoring.
But Chesterfield grabbed the lead against the run of play when substitute Armand Gnanduillet headed home Conor Townsend's corner in the 44th minute.
And from the restart the visitors doubled their tally, Richards on the end of a long ball before firing a stunning 25-yard effort which was too good for Scott Davies - capping a stunning finish to the first half.
Richards then made it three-nil just after the hour mark, lashing home the rebound after Davies saved Sam Togwell's goalbound effort.
Andy Mangan's first goal since August gave the home side hope, a perfectly timed run to get on to David Ball's defence-splitting pass and a cool finish.
Had Mangan found the net again, first shooting wide from 18 yards before then looping the ball over the bar from close range, Fleetwood might have set up a grandstand finish but it was not to be.
As it was Chesterfield stood firm and are certain to finish above the home side, who are winless in five, but Paul Cook's side cannot now catch Bradford in the fourth and final play-off place.
Alan Knill's side claimed a second successive win to move clear of the relegation zone and climb up the League Two table.
The Gulls were given a helping hand when they took the lead with a freak goal in the 28th minute. Rene Howe and Stewart Drummond challenged for the ball with the Morecambe defender passing back to his own goal only to see home keeper Barry Roche let the ball crawl under his feet and into the net.
The goal was an added blow for the Shrimps who found themselves a man down in the 23rd minute when teenage debutant Chris Doyle was shown a straight red card for hauling down Elliot Benyon.
To their credit Morecambe hit back with Ryan Williams testing Michael Poke with a dipping, swerving 40-yard free-kick.
The home side maintained the pressure at the start of the second half with defender Andy Wright hitting the post with a superb volley from the edge of the area.
Kevin Ellison then went close with a 30-yard volley that flew inches wide of the left-hand post before the Gulls doubled their advantage through Ashley Yeoman.
The substitute had only been on the field for a matter of seconds when he broke down the left and fired a low left footed shot past Roche.
Williams went close to adding a deserved consolation with an injury time free-kick which was excellently saved by Poke.
Hawley tapped in the only goal of the game four minutes before the break to move the Iron to within three points of safety as the Dons' play-off hopes were ended.
The Dons dominated possession in a low-key first half, but lacked a cutting edge, with Patrick Bamford scuffing two chances straight at goalkeeper Sam Slocombe.
Jon Otsemobor also headed over Shaun Williams' corner from close range and Luke Chadwick drilled a shot just wide from the edge of the box.
But Scunthorpe were dangerous on the break and Mark Duffy had a low drive pushed wide by Ian McLoughlin and Paul Reid headed against the crossbar before Hawley bundled in Jimmy Ryan's low cross to put the visitors ahead in the 41st minute.
The Dons piled forward after the break, but Slocombe produced a string of fine saves to earn the Iron victory, acrobatically denying Williams, Chadwick, Dean Bowditch and Mathias Doumbe.
Scunthorpe almost doubled their lead late on when Anthony Forde blasted against the post and Hawley was denied by the boot of McLoughlin.
Dundee arrived in Paisley 13 points behind their opponents with five games to play but claimed all three points to keep them up for now.
Jim McAlister had smashed the visitors ahead only for Saints' top scorer Steven Thompson to level just after the interval.
With time running out, substitute Carlo Finnigan squeezed a header down off the underside of the bar and just over the line to ensure their battle against relegation continued to next week's home fixture with Hearts.
Talking of the Jambos, John Sutton's early goal proved enough to put them firmly in the running for seventh place, although they made life difficult for themselves in a 1-0 win at Kilmarnock.
Sutton scored the game's only goal from close range in the fifth minute and Hearts were rarely troubled before Cammy Bell denied them a second when he saved Ryan Stevenson's 62nd-minute penalty.
But they had to survive some late pressure with Kris Boyd passing up two good chances for the hosts.
Meanwhile, Partick Thistle claimed the Scottish First Division title with a deserved 2-0 victory at Falkirk.
The visitors, seeking a return to top flight Scottish football for the first time in a decade, sealed their promotion thanks to second-half goals from Andy Dowie and Chris Erskine.
Defeat for the Grecians ended their hopes of making the play-offs but the fourth-placed Robins can still go up automatically with a win over Bradford on the final weekend provided Rotherham lose at home to bottom club Aldershot.
The only goal came inside five minutes as Sido Jombati's deep free-kick was headed back across goal and fell on the edge of the box to Penn, who fired a low shot into the far corner.
Exeter's response was good with Jamie Cureton and Jamie Reid going close. Cureton missed a glorious chance when he nipped in behind the defence from Tommy Doherty's quickly taken free-kick but he snatched at his shot and pulled it wide.
Michael Hector went close to a second for Cheltenham but his header was saved by Artur Krysiak.
Despite the need for two goals to keep their play-off hopes alive, the Grecians never threatened until late on in a terrible second half.
Substitute Jimmy Keohane had their best chances but he blazed over the bar from 18 yards and was then denied by a brilliant fingertip Scott Brown stop.
