Paolo Di Canio blamed a 'silly mistake' for Swindon's exit from the Johnstone's Paint Trophy at the hands of rivals Oxford.
Last season's runners-up lost 1-0 at the Kassam Stadium, with substitute Alfie Potter scoring in the 88th minute for the hosts following a defensive mix-up.
A lack of communication between Darren Ward and Aden Flint resulted in both players ending up on the floor as the ball ran loose to James Constable. He whipped in a cross from the left and Potter made no mistake from close range, firing under goalkeeper Wes Foderingham.
New signing Giles Coke, Raffaele De Vita, Adam Rooney, Ward and Simon Ferry all wasted good chances in the first half for Swindon, while Federico Bessone, Rooney and substitute John Bostock squandered opportunities after the break.
"We didn't concede a shot on target and we gifted them one mistake to win the game. One silly mistake has given them the opportunity to score and it finished the game," said Robins boss Di Canio.
"We lost because of a stupid moment. If it was at the beginning there's obviously time to recover, but it was impossible that late on."