Swindon manager Paolo Di Canio admitted his players weren't able to handle playing in a derby as they crashed 2-0 at local rivals Oxford.
The npower League Two leaders' 10-game winning run was ended by a fiercely committed and competitive Oxford side, who overcame the early dismissal of striker James Constable to register a victory against the odds.
Constable was sent off in the 11th minute for elbowing Joe Devera, but the U's scored twice in quick succession through Asa Hall and Oli Johnson, with the Robins' travails against the 10 men petering out in the second half.
Di Canio said: "My players showed me they're not ready to play in this kind of game because we didn't really create any chances in the second half. That doesn't mean we're a bad team. But I expected a lot more from our main players. Something happened to them in their brain and they weren't able to deal with it.
"They should have been helping the younger players in the team, but for me it's a step forward to understand this. We had three crazy minutes and unfortunately it's against the big rivals for our club and our fans.
"But even after this defeat we stay at the top of the table. We are stupid if we take this game into the next ones coming up. We have two home games next and we need to forget about this now."
Oxford manager Chris Wilder couldn't hide his delight as his team bounced back from their disappointing home draw against Macclesfield last week with a win which sent their supporters into raptures.
Wilder said: "It was a massive effort from my players because Swindon are a real good side.
"We rode our luck at times, and we were indebted to some great defending and great goalkeeping, especially not only going down to 10 men but going down to 10 men that are not regulars in the side.
"I would like us to be a lot closer to Swindon in the league. To my mind I have no doubt they will go up."