The Tangerines had looked to be sailing into the next round, but threw away the match by gifting two cheap goals to MyPa-47 and sent the huge home support devastated.
It started so well for the Terrors and they immediately set out their stall of attacking down the flanks and drawing the visitors in, but unfortunately they lacked that cutting edge in front of goal.
That was until the 15th minute when Mark Kerr broke the deadlock. Barry Robson's curling free-kick was headed out to the midfielder, who standing at the edge of the box, smacked in an unstoppable shot past the diving keeper.
The hosts doubled their lead in the 29th minute courtesy of some attacking left-wing play from Stuart Duff and Robson, with the latter's cross headed on by Jim McIntyre and knocked in by Collin Samuel at the far post.
Only a tremendous save prevented Lee Miller's 37th-minute wonder header going in the postage stamp as United totally dominated - although MyPa created the odd flutter in the home defence.
It was going so well, but in the second period United lacked the same drive and momentum.
Despite this, they still created opportunities that should have put the game beyond any doubt.
In the 68th minute Kerr agonisingly headed off the face of the crossbar after excellent build-up play from McIntyre.
And United were made to pay for their slackness, when MyPa were awarded a penalty in the 75th minute. Duff's reckless challenge was adjudged to be a foul and Munoz Adriano made no mistake in converting.
And it went from bad to worse for the Tangerines when amazingly MyPA scored the killer second in the 82nd minute.
A long ball was badly judged by Alan Archibald allowing Adriano, who looked to be in an offside position, to lob the ball over the advancing Derek Stillie and send the hosts crashing out.