United had ridden their luck throughout a poor display in which they needed goals from a couple of set pieces to put them ahead midway through the second half.
But they were unconvincing in attack - despite a boost provided by new signing Jon Brady from Hereford - and they never looked comfortable at the back from the moment Rovers shocked them with a third minute goal.
That came from centre-back Jon Richardson who easily out-jumped the defenders to nod the ball under the crossbar from close range.
It was a corner which produced the equaliser in the 18th minute, Andy Duncan powering a header home after beating the defence to a perfectly flighted cross from Brady.
But Cambridge, hoping to carry on their excellent home form and move up towards the play-off places, were never really able to take control in a scrappy game.
Rovers should have regained the lead before the break but Ben Harding and the impressive Guy Madjo each fired shots wide.
Cambridge went in front in the 67th minute thanks to a needless foul from Damon Searle producing a free-kick which gave Fola Onibuje the chance to head in at the far post.
But the response from Rovers was to go all out to get something out of the match and they piled on the pressure in a hectic final 15 minutes.
It looked as though it would still be United's day as Richardson twice headed inches wide with keeper Scott Howie stretching for the ball.
But in the third minute of stoppage time when Richardson again got to the ball first and headed towards goal, Howie could only palm the ball down for substitute Bruno Teixeira who headed straight back past him to secure the point.