POOR defending from set-piece situations cost United all three points on an incredibly frustrating night at Dover Athletic.
The Gulls were coasting in the first-half and should have been more than a Krystian Pearce goal ahead at the interval, but Dover came back to equalise with Connor Essam notching from a corner.
Substitute Ashley Yeoman restored the Yellow lead and it looked like a massive win was in the bag, but another corner in the dying seconds again caught United out, and Tyrone Sterling was on hand to level for a dramatic Dover point.
United manager Chris Hargreaves stuck to the same starting line-up and bench that came through a bruising weekend win over Eastleigh at the Launa Windows Stadium.
Dover boss Chris Kinnear made two changes to the side that achieved a superb 2-1 win at Gateshead on Saturday, with the defensive duo of Richard Orlu and Sam Francis replaced by Callum Davies and Sean Raggett.
Against a Dover side boasting an incredible run of eight games without conceding at home, the Gulls immediately found space in the behind for Ryan Bowman to force Andy Rafferty into a fine early save and the rebound only just evaded the onrushing Jordan Chapell.
Louis Briscoe was the next to find space and he quickly shifted the ball back to Bowman, who was pulling the trigger just as the Dover defence recovered to make an excellent block-tackle.
United were well on top and the confidence of a fine weekend win was coarsing through Yellow veins. Toby Ajala was the new threat for the Whites, cutting inside to unleash a fierce drive that flew inches over the bar.
Dover were adopting a fairly industrial strategy of pumping balls into the Yellow penalty area and it was all fairly comfortable stuff for United’s defensive partnership of Aaron Downes and Pearce.
The Gulls were in control, however, and the opening goal was fully deserved when it arrived. A corner from Luke Young caused completely mayhem in the home box and when the ball popped up into the air, Pearce was in the right place to nod home his first of the season.
United were soon breaking forward in search of a second, as Briscoe latched onto a Downes clearance and fed the ball through for Chapell, but the young winger was unable to find the target with only Rafferty to beat.
The home side finally found some impetus in their attacking play 10 minutes before half-time and Nicky Deverdics had goalkeeper Martin Rice looking anxious for the first time in the evening, as a fizzing drive narrowly missed the target.
The Gulls were soon back in the ascendancy and a clever lobbed pass from Chapell looked like releasing Ajala for a run on goal, but Essam saved his side with a fine recovery challenge.
A wicked cross from Luke Young then completely deceived Rafferty and he was left as a very relieved onlooker as the ball grazed his post, falling neatly into the goalkeeper’s hands in the final chance of a first-half completely dominated by those in Yellow.
Dover had to improve in the second-half and United full-back Durrell Berry did well to repel an early surge. Rice then made a super adjustment to palm clear a deflected shot from Soloman Taiwo.
Ajala relieved the very mild spell of home pressure, bursting down the left and looking inside for Bowman, who was crowded out by a Dover defence desperately retreating toward their own goal.
The next chance was beyond gilt-edged, as Bowman was sent clean through on goal and he showed terrific composure to skip past Rafferty, but the final shot from a tight angle was brilliantly cleared off the line by Essam, who had the presence of mind to cover in behind his goalkeeper.
Dover, in fairness, were looking a far more dangerous outfit and a fantastic sweeping cross from Sterling only just eluded the back-post run of striker Tom Murphy, as the home side cranked up the pressure.
The Gulls were happy to keep things scrappy for the next few minutes, taking the sting out of the Whites and a commanding performance from Downes was proving immovable barrier for the Kent outfit.
Dover did have a sniff at goal when a loose ball in the box fell to Chris Kinnear jnr but he sliced a horrible shot miles wide, prompting Hargreaves to freshen up his attack by removing Briscoe for Yeoman.
Courtney Richards then came to the fore, twisting himself into space for a fierce drive that had Rafferty diving full-stretch to turn the ball wide. A decent effort from the young midfielder, who was enjoying a good night on the Kent coast.
Dover responded by replacing Deverdics with James Poole and he was soon celebrating with his captain, as the substitute’s whipped corner picked out a completely unmarked Essam for the equaliser and a horrible way for United to concede.
Defender Angus MacDonald replaced Chapell in the next Yellow change in a bid to add some height to the United defence, as Dover continued their aerial assault on the Gulls. Rice then got down quickly to thwart a Murphy effort.
From nowhere, United broke straight up the other end and a defensive slip from the Whites presented Yeoman with the sort of chance any striker would dream about, as the substitute prodded past Rafferty after his first shot was blocked.
The points were far from won, however, and a rampaging Rice came storming off his line to grab a ball over the top, but Kinnear jnr got their first. Fortunately, the Yellow custodian was back in his goal quickly enough to tip the ball over.
James McQuilkin came on for Ajala in the final Yellow change and with five minutes left on the clock, it was time for the Gulls to keep the ball and protect their valuable lead. Dover, meanwhile, removed Kinnear jnr for Greg Rutherford.
The closing seconds were full of tension for United and Berry was the hero with a fine clearing header under heavy pressure. Dover had won final corner, and disaster struck.
The home side had looked a danger from set-pieces all night and massive disappointment for the Gulls, as the defence failed to clear, giving Sterling the room to lash home a painful leveller.
Dover (5-3-2): 30 Andy Rafferty; 29 Connor Essam (capt), 18 Callum Davies, 5 Sean Raggett, 26 Tom Bonner, 16 Tyrone Sterling; 17 Nicky Deverdics (31 James Poole 73), 4 Chris Kinnear jnr (21 Greg Rutherford 86), 28 Soloman Taiwo; 10 Tom Murphy, 14 Stefan Payne. Substitutes (not used): 3 Tom Wynter, 8 Liam Bellamy, 11 Barry Cogan.
United (4-4-2): 1 Martin Rice; 29 Durrell Berry, 5 Krystian Pearce, 4 Aaron Downes, 25 Levi Ives; 12 Jordan Chapell (21 Angus MacDonald 78), 8 Luke Young (capt), 16 Courtney Richards, 10 Toby Ajala (22 James McQuilkin 85); 7 Louis Briscoe (14 Ashley Yeoman 68), 9 Ryan Bowman. Substitutes (not used): 23 Jordan Seabright (GK), 24 Duane Ofori-Acheampong.
Referee: David Rock