Two quickfire goals from Luke Berry and Barry Corr condemned Barnet to an undeserved defeat at The Abbey Stadium.
John Akinde opened the scoring in the 67th minute after a period of dominance from the Bees.
But 74th and 75th minute goals from the home team sent Martin Allen’s side home empty-handed.
Barnet began the first half strongly, but it was the home side who had the first clear-cut chance of the match.
Leon Legge headed a 13th minute Cambridge corner at goal, but a crucial touch by Gavin Hoyte ensured that his effort cannoned off the crossbar.
Barnet responded by hitting the bar themselves in the 24th minute – Aaron McLean’s long-range shot rattling the frame of the goal after a good period of pressure from the Bees.
Andy Yiadom then had a chance to give Barnet the lead just before half time.
The captain connected with Sam Togwell’s long throw into the box, but put his shot wide of the right post.
It was Yiadom who had the Bees first effort of the second period when his shot from the edge of the area was deflected wide after 57 minutes.
Two minutes later Barnet took the lead.
Tom Champion split the Cambridge defence with a wonderfully weighted pass, and Akinde raced through to slot the ball past Sam Beasant and into the net.
Akinde could have extended Barnet’s lead when he found himself one-on-one with the keeper in the 67th minute, but the striker’s chip floated wide of the right post.
Cambridge then turned the game on it’s head with two goals in a minute.
First Berry headed home a Harrison Dunk cross, before Corr drilled a right sided shot past Ian Lawlor with a quarter of an hour left on the clock.
The visitors pushed hard to get back into the match but were unable to find an equaliser – Barnet now travel to Northampton Town on January 2nd.
BARNET: Lawlor, Hoyte (Lisbie 76’), Johnson, N’Gala, Dembele, Togwell (Muggleton 74’), Weston, Champion, Yiadom, McLean (Gash 54’), Akinde.
Subs Not Used: McKenzie-Lyle, Batt, Stevens, Odoffin.