Chris Wilder was left to reflect on Oxford's profligacy as his side fell to a disappointing 1-0 home defeat to lowly Bristol Rovers.
It was Rovers' first away win of the season and knocked Oxford from top to second in the League Two table.
John-Joe O'Toole grabbed the winner from the penalty spot with 71 minutes on the clock after Jake Wright tripped Eliot Richards.
A few minutes prior to that, James Constable had missed a golden opportunity for Oxford when he headed over from five yards.
U's boss Wilder said: "We were architects of our own downfall with the missed chances.
"We had the best three chances of the game. They were three ridiculously good chances.
"They (Rovers) started better than us but we then did start to create some good opportunities. But we're disappointed in ourselves that we've not reached the standards we have been lately.
"My first reaction with the penalty decision was that it was definitely outside the box, and I've seen replays of it which show it was. But the referee wanted to steal the show and he wanted to give the penalty."
Oxford's miserable afternoone was compounded by the sight of winger Alfie Potter being stretchered off at the end after a foul by Tom Lockyer.
Wilder said: "My biggest concern, more than the defeat or the performance, is Alfie. It was a poor challenge.
"We thought it might be a break but it looks like ligament damage to his ankle, which is the last thing we need."