Dennis Oli scored on his debut for Cambridge and should have been the hero of a hard-fought victory for the fourth from bottom side against high-flying Mansfield.
But although Cambridge dominated the match, twice taking the lead, they were undone by two basic blunders from goalkeeper Shaun Marshall.
Signed as an emergency replacement on the eve of the match from Swansea City, Oli took only 17 minutes to make his mark.
He was in the right place at a corner kick when Adam Tann's header bounced down off the crossbar.
With Justin Walker running midfield Cambridge were in control, but two minutes before the break Marshall could not get to a Scott McNiven free-kick, allowing Colin Larkin to tap in from close range.
Nine minutes into the second half Oli raced away down the right wing and hit a perfect cross to Jermaine Easter whose shot bounced in of the right-hand post.
Cambridge rode their luck when a Neil Mackenzie piledriver struck the bar, but Mansfield's second equaliser was not long in coming.
Marshall fumbled another McNiven free-kick and the ball dropped to Mackenzie who rammed it home.
Cambridge had chances to hit back and win in the later stages but Easter and Daniel Chillingworth fired wide from good positions.