United got the first break five minutes into the second half when Mark Wilson swung in a menacing corner which goalkeeper Jamie Langfield failed to read. As he missed with a punch, he left his goal gaping and John Souttar was able to head home the opener.
Aberdeen levelled 15 minutes later though as referee Collum ruled United left-back Andrew Robertson had used an arm in his own box when Barry Robson's through-ball skipped up off the turf and hit his armpit.
Robson blasted home the penalty and insult was added to Arabs injury when Peter Pawlett stooped low to head home a late winner after Radoslaw Cierzniak - the hitherto impressive United goalkeeper - allowed Robson's free-kick to spill from his grasp.
He said: "I spoke to Andrew and he thinks the ball hit him on the chest but the referee made the decision quickly. Sometimes they go for you and other times they don't.
"We're disappointed because it was a frustrating way to finish the match. There wasn't much quality in the game, it was a lot of free-kicks and stop-start all the time.
"I thought it would be someone's mistake which would decide the game.
"We maybe didn't deserve to score and go in front when we did but there wasn't a lot of quality in the game.
"We've now lost three in a row but I wouldn't say it was a concern, the last two games were different altogether because we were down to 10 men against St Mirren and St Johnstone
"I was disappointed with some of our passing. We handled most of the pressure Aberdeen put on us but it was a poor game."
- Sports & Recreation
- Barry Robson
- Jamie Langfield
- John Souttar