Watch highlights of Oxford United's 2-1 defeat at Mansfield.
Oxford United were sent crashing to their first defeat of the season by mid-table Mansfield at Field Mill.
The runaway Blue Square Premier leaders were beaten by two Stags goals in the space of just four first-half minutes.
Skipper Ryan Williams scored the first from the edge of the box before setting up striker Jake Speight for number two.
Oxford top scorer James Constable pulled one back with his ninth goal of the season to give the leaders hope - but they failed to add to it.
Mansfield started brightly, winger Louis Briscoe having an early effort tipped round the post by Oxford keeper Ryan Clarke before they took a deserved lead.
Clarke made a good save to keep out Speight's powerful drive but, when the ball came back, Williams lobbed high into the roof of the net - the first goal Oxford and Clarke had conceded in 561 minutes.
The duo then combined again to double the Stags' lead.
Williams delivered from the left and Speight allowed the ball to run across his body before planting a shot into the top corner.
Oxford forward Cook came close to pulling one back before the break only for his superb 30-yard effort to come back off the bar.
And the leaders improved still further after the interval, getting one back when Adam Murray's cross was dispatched clinically by Constable. But Mansfield held on for a fine win.
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