Barnsley's players celebrated and Sheffield United's looked absolutely crestfallen following this eventful and potentially pivotal match at Oakwell.
Goals from Gary Gardner, Oliver McBurnie and substitute Tom Bradshaw kept the hosts' hopes of preserving their Championship status alive.
Chris Wilder's side, who had earlier appeared destined to secure the win which would have seen them climb into the top six, found themselves condemned to an anxious afternoon in front of the television waiting to see how Middlesbrough, Millwall, Bristol City and their other rivals for play-offs got on.
When John Fleck and Leon Clarke pounced following Gardner's opener, United sensed the derby spoils were their's. Wilder's reaction on the bench, when McBurnie and then Bradshaw turned the contest on its head, confirmed it should have been. To his right, Morais celebrated wildly, knowing Barnsley had just grabbed a lifeline in their battle for survival.
Wilder had earlier sprinted down the touchline to make sure he was already in the dressing room when his players arrived following Gardner's strike midway through the first-half. Exactly what was said during the interval remained behind closed doors. But, given the manner of United's display during the opening 45 minutes, it was unlikely to be polite.
Morais' men deserved their lead and, with the visitors falling so far short of the standards they had set against Brenford and Cardiff City over the Easter period, will have been disappointed not to have built an even bigger advantage.
Barnsley's failure to do so eventually proved costly when Fleck claimed his second goal of the season. Like Gardner's before, a United breakthrough had been coming following their fast, furious and much more purposeful start to the second period.
Neither, so great was the momentum they enjoyed following the midfielder's intervention, was it a surprise when Clarke turned the fixture on its head. The defending, after his initial shot had been smothered, was woeful. But the finish, as Barnsley's goalkeeper desperately tried to scramble back into position, suggested a player in peak form and confidence.
Another, Scotland international McBurnie, restored parity and set-up a furious finale as both United and their neighbours chased the three points they craved. Bradshaw ensured it was Barnsley who grabbed them after escaping his marker in the box with only two minutes of normal time remaining.
It was a measure of both manager's satisfaction with their respective team's performance levels in recent weeks that the only change to either's starting eleven was enforced.
United, who named Jake Wright at centre-half in place of the injured Richard Stearman, had been warned to expect an early barrage from their neighbours who are locked in a battling for survival at the opposite end of the table. The message, issued by Wilder before Thursday's penultimate training session at the Steelphalt Academy, proved to be correct with Kieffer Moore quickly emerging as a threat. Indeed, the former Ipswich Town striker should have headed Barnsley into a second minute lead when, after winning an early corner, he then glanced a clear header wide.
United escaped again when George Moncur hit the crossbar from just inside the box after teasing George Baldock and Chris Basham.
United's first move of note, which should also have led to a goal, came midway through the first-half. A quick-fire interchange and sharp one/two involving Leon Clarke, Lee Evans and John Lundstram ended with the latter slicing high and wide. The engineering was perfect. The execution of the finish, however, was anything but. It proved a costly mistake when, in the 25th minute, Gardner fired Barnsley in front with a powerful shot into the top left hand corner of Simon Moore's net.
Goals do not just change games, they also change the mood of crowd too. Suddenly Oakwell, which for the most part had been reverberating to the sound of famous United anthems, sparked into life at the other end too.
As the groundstaff applied a new coat of paint to the walls of United's dressing rooms, Wilder's side made a much more positive start to the second period. True, their determination to press forward left them vulnerable on the counter-attack but Barnsley suddenly looked decidedly uncomfortable. Particularly when Fleck dragged United level in the 57th minute with another powerful strike; albeit from much closer range than Gardner.
In the 65th minute, Clarke's 17th goal of the season and second in as many outings moved them in front. His initial attempt was well blocked by Davies but, when Barnsley failed to clear, United's leading goalscorer did not need a second invitation to smash the ball into the back of the net before celebrating wildly in front of the travelling support. Nine minutes later, though, the hosts were level when Moore parried substitute Adam Hammill's attempt and McBurnie tapped home. Bradshaw followed suit to send the home fans wild and United into despair.
Barnsley: Davies, Yiadom, Jackson, Lindsay, Fryers, Potts, Gardner, Moncur (Bradshaw 69), Isgrove (Hammill 60), Moore, McBurnie (Cavare 89. Not used: Walton, Mallan, Pinnock, Mahoney.
Sheffield United: Moore, O'Connell, Wright (Wilson 88), Basham, Stevens, Baldock, L Evans (Sharp 39), Fleck, Lundstram, Brooks (Duffy 48), Clarke. Not used: Eastwood, Leonard, Lafferty, Holmes.
Referee:Scott Duncan (Lancashire). Star Man: John Fleck. Attendance: 16,041