United's Under 18s fell to a thumping 5-1 defeat at Burnley despite being well on top with the game at 1-1 for 65 minutes.
But four goals in seven crazy minutes undid all of United's hard work and left them with an unclimbable mountain.
Coming into the game off the back of three successive clean sheets, United did well in the opening stages and restricted Burnley to only half chances.
It was on the 20-minute mark the hosts took the lead when Rain's corner found its way all the way to the far corner and into the net.
But it took just 15 minutes for United to respond as some good, incisive football led to a defender handballing Tom Williams' cross and Reon Potts stepped up to slot the resulting penalty home.
From then on, United took control of the game but couldn't find the vital second. Andre Brooks saw an effort go wide and Potts was denied with a close-range header before Frankie Maguire saw his long-range effort tipped over.
It was a counter attack with just over 20 minutes to go which was the start of United's collapse. Conn-Clarke fired home to put Burnley back into the lead before George added a third to make it two goals in as many minutes for the hosts.
Conley added a fourth from close range as Burnley started to run riot and a fifth came soon after when substitute Jaydon Major slammed home another for a score line which certainly wasn't reflective of the Blades' performance.
Blades: S. Kelly, Weaver (T. Wilson 26), Brooks, Maguire, Foulstone, Gomis, Williams, G. Wilson, Potts, Ompreon (Boyes 74), Gaxha (Viggers 74).
Unused: Hiddlestone, Neal.