Shane Tudor curled in a dramatic injury time equaliser to edge Cambridge closer to safety and put a huge dent in Mansfield's automatic promotion hopes.
Mansfield substitute Lee Williamson fouled Justin Walker on the edge of the area and from the last kick of the match Tudor bent his free kick over the wall and into the bottom corner.
It was rough luck on promotion-chasing Mansfield, who looked to be cruising to their second win in five days at a wind-swept Field Mill.
They had survived a scare in the 54th minute when Tony Vaughan raced back to clear off the line after Jermaine Easter had rolled his finish from the edge of the area past stranded keeper Kevin Pilkington.
Then in the 61st minute the Stags grabbed the lead. Pilkington's long kick cleared the defence and Junior Mendes held off Andy Duncan before finishing clinically from six yards.
With new manager Claude Le Roy again absent through his French TV duties, his assistant Herve Renard took charge for the second successive Saturday.
And the U's almost grabbed a lucky 11th minute lead. Stuart Bimson's high cross swirled in the strong wind, dropped onto the bar, and hit Mansfield full-back Vaughan before a grateful Pilkington saved on the line.
Mansfield, struggling against the near-gale, finally created a chance in the 18th minute.
Colin Larkin's clever turn and cross picked out Mendes, but his strike partner scooped over from six yards. United, looking extremely dangerous on the break, created the better first half chances with the wind at their backs.
In the 25th minute Walker's 20 yard snapshot flew narrowly wide.
Then five minutes before half time Luke Guttridge's dipping 25 yard volley from a partially-cleared corner was acrobatically tipped over by the flying keeper.
Mansfield upped their tempo in the second half and Wayne Corden's 20 yard curling free kick was deflected, past the post as they began to dominate.