Port Vale boss Micky Adams admitted his players failed to live up to the standards they have set this season in their 2-1 defeat at Oxford.
Vale had been riding on the crest of a wave after winning their last six games in all competitions but they came unstuck against Chris Wilder's side.
Leaders Vale looked to have secured a point when Gareth Owen cancelled out Tom Craddock's strike in the first minute of time added on.
But Craddock had the final say with a last-gasp winner, much to Adams' dismay.
He said: "It's definitely a shock to get two goals against us. We've had an off day. Individually and collectively we've not got it at all right.
"We were on the back foot from the first whistle. But full credit to Oxford, they showed a lot more desire and determination than we did. And when we've managed to get back into it right at the death, we've then contrived to lose it."