Second-half goals from Jai Reason, Sean Marks and Ben Wright gave Braintree a win over promotion-chasing Cambridge.
The hosts took the lead when Reason's free-kick eluded everyone and found the bottom corner.
Marks added a second from just inside the box before Wright's lob appeared to kill the game off.
But James Jennings scored a solo effort with two minutes left and Tom Shaw's goal deep into stoppage time gave the result respectability for the U's.
VIEWS FROM THE DRESSING ROOM
Cambridge United boss Jez George told BBC Radio Cambridgeshire:
"The only thing we need to forget from this afternoon is our two goals because they put a false gloss on the scoreline.
"The rest of it needs to be kept at the forefront of our mind because it wasn't good enough from start to finish.
"It looked like men against boys out there and we just couldn't handle them."