Rock-bottom Exeter's dream start ended in a nightmare as Cambridge youngster John Turner made a sensational start to his League career.
A Cambridge side needing nothing less than victory to stay in the play-off race found themselves a goal down after only 22 seconds.
Winger James Coppinger hammered a cross into the box where centre-back Izzy Iriekpen sent a powerful header over his own goalkeeper and under the crossbar.
But more than 90 minutes later there was an equally amazing end to the match when 17-year-old Turner, a 76th minute substitute, ran onto a last-gasp drop-out from keeper Shaun Marshall, controlled the ball expertly and rammed an angled drive inside the far post for the winner.
Cambridge had dominated the game, creating a host of first-half chances, but needed a 69th minute penalty from skipper Paul Wanless to draw level after David Kitson was harshly judged to have been pushed by Christopher Todd.
Exeter squandered a glorious chance to get what might have been a decisive 2-0 lead when their top scorer Steve Flack stubbed a shot straight at Marshall from only four yards out.
Said Cambridge boss John Taylor: 'We deserved to win but looked like running out of time after wasting our chances and the kid has kept us in it.
“It was no gamble sending him on because we know he can get a goal in any situation.†Exeter manager Gary Peters warned: “We won’t stay in the league if we cannot finish better than this.
“We matched Cambridge but got nothing after wasting some really clear opportunities.â€