West Ham were left cursing their luck after an injury-time equaliser denied them victory in an amazing evening of football at Bramall Lane.
One-nil down after five minutes, three-one up at half-time, Alan Pardew's men endured a roller-coaster last 20 minutes as Sheffield United claimed a controversial goal to get back in it, saw young goalkeeper Stephen Bywater pull off a magnificent penalty save to deny them an instant equaliser and then finally drew level three minutes into added-on time after Hammers had missed a couple of glorious chances to kill off the game.
It meant yet another draw for Pardew's team and yet another lead thrown away, but there were at least some positives for Hammers' loyal travelling fans to take home.
After promising to make changes in the wake of the disappointing home defeat at the hands of Preston last week, the Hammers boss stayed true to his word and introduced an almost entirely new back four in an effort to revitalise his troops.
New boys Harley and Andy Melville - signed in the swap deal that saw Ian Pearce head in the opposite direction - came in at left-back and centre-half respectively and Anton Ferdinand replaced Robbie Stockdale at right-back for his first start since September.
With Bywater taking the place of the departed David James, that left Christian Dailly as the only part of the rearguard to retain his place - and indeed the only legacy of the West Ham United team that endured the agony of relegation from the Premiership at Birmingham City on the final day of last season - as Pardew's new-look side attempted to get the promotion push back on track.
However, after just five minutes, it seemed like a case of same old story as the hosts tore through Hammers' back-line and opened the scoring.
Paul Peschisolido collected Wayne Allison's flick-on and fed Montgomery, whose low cross from the right returned the favour for the Canadian striker, who stabbed the ball past a helpless Bywater.
Moments later, it could have been two, as Etherington's weak back-pass dropped straight at the feet of the goalscorer, but Hayden Mullins thankfully got back in time to cut out the danger.
That proved to be something of a wake-up call for Hammers, who began to find their passing rhythm in midfield and put the Blades' defence under pressure. 14 minutes after conceding the opener, Hammers were level - thanks to a first goal of the season from Michael Carrick.
Collecting possession midway inside the opposition half, the Geordie midfielder swapped passes with David Connolly before skipping into the box and sweeping the ball confidently past goalkeeper Paddy Kenny.
Just three minutes later, it got even better for the visitors. Having already made an impressive start on the left-hand side, Harley made it a dream debut as he cut in from the touchline and advanced to the corner of the penalty area before unleashing an unstoppable, bending shot that whistled past Kenny and into the top corner.
The former Chelsea youth starlet looked almost stunned as he celebrated his strike against the team with whom he spent two spells on loan last year - but certainly not as stunned as his former club-mates.
Seven minutes before half-time, Harley turned creator as Hammers made it three, lifting a clever ball over the defence to release Etherington, whose looping cross was superbly volleyed in by Harewood at the far post.
As they headed off at the break, the visitors must have been confident of making it a third successive victory on their travels, but the sight of goalscorers Carrick and Harewood sharing a 'high-fives' celebration as the strode down the tunnel may just have added a touch of extra motivation to Neil Warnock's half-time team-talk.
Despite pushing more men forward after the restart, though, the Blades didn't really suggest that a comeback might be on the cards until midway through the half, when Warnock introduced debut striker Paul Shaw as a substitute.
The former Arsenal and Gillingham front-man added more presence to the United attack and popped up to drag his new side back into it just eight minutes after his arrival on the pitch - although Hammers could consider themselves unfortunate not to have seen the goal ruled out following a clear foul on Ferdinand.
As he cleared a looping ball into the box, the young defender was left grounded after being caught by the boot of Peschisolido as he attempted an acrobatic overhead kick. However, referee Paul Durkin waved play on and the ball found it's way out to Shaw, who fired low under the dive of Bywater.
The 22-year-old goalkeeper - playing his first senior match for the club in almost three years - might have been disappointed with his failure to keep that one out, but he certainly responded in spectacular fashion just two minutes later.
After Ferdinand's clumsy and needless challenge on Peschisolido that gave Durkin no choice but to point to the spot, Michael Tonge's fiercely-struck spot-kick seemed destined for the net until Bywater somehow stretched out his left-hand to powerfully divert the ball clear.
The England under-21 starlet was mobbed by his relieved team-mates, who really should have ensured that he remained the hero by finishing off their opponents in the final stages.
Most guilty was Etherington, who twice wasted golden opportunities to set up a fourth killer goal on the counter-attack. First he shot straight at Kenny from an acute angle rather than hand Connolly or Deane an open-goal, then he curled a weak effort at the Blades keeper after being released by Connolly.
Still it looked as if Bywater would steal the limelight, though, as he produced another wonder-save to deny substitute Dean Sturridge from just four yards out, but his ecstasy turned to agony in the dying moments when, rushing out to clear a deep corner he was never going to reach, he was left stranded as Phil Jagielka looped a header into the net.
What would normally have been considered as a decent point away from home at one of the favourites from promotion, felt almost like a defeat as Hammers failed to close the gap on Warnock's third-placed side.
Still, with the possibility of more new faces to arrive at Upton Park before next week's FA Cup fourth round trip to Wolves, Alan Pardew's rebuilding process is now fully underway and will hopefully begin to bear fruit in the coming weeks.