Veteran striker Steve Claridge spared Millwall's blushes in the 1-1 draw at Third Division Cambridge.
His goal midway through the second half against one of his former clubs earned a replay the visitors hardly deserved.
Only resolute defending and good goalkeeping by Tony Warner kept Millwall in this cup-tie.
Cambridge were clearly not overawed by a team two divisions above them and attacked from the start.
Shane Tudor's fierce 20-yard shot was only parried for a corner by Warner and then the same player's shot from 25 yards lacked both power and accuracy, while David Bridge's 25-yard shot just went over the bar.
Bridges tested Warner with a curling shot from the left that the Millwall keeper did well to hold, Paul Wanless contrived to head over the bar from a couple of yards and then Warner pulled off a fantastic save from Tudor.
Daniel Chillingworth put a fierce shot just wide and Tom Youngs hit the upright with a well-struck effort.
And as frustration got the better of Millwall, Ronnie Bull was booked for dissent on the stroke of half-time, when, amazingly, the score was still 0-0.
Cambridge started the second half as they finished the first and took a deserved lead four minutes after the restart. Warner pulled off another fine save from a powerful Adam Tann header but from the resulting corner Youngs headed the ball home from close range.
David Livermore became the second Millwall player to be booked as the First Division continued to struggle.
Just when it looked as if Cambridge would grab a second, Millwall drew level in the 60th minute with their first serious attack of the half.
Claridge got behind the home defence to prod home a Paul Ifill cross from close range.
Millwall's Paul Robinson became the third Millwall player to be booked after a foul on Fred Murray and Cambridge's Adam Tann also received a yellow card for a foul on Steven Reid as both sides went looking for a winning goal.
Shaun Marshall, who had very little to do in the Cambridge goal, pulled off a good save from Charley Hearn and then gathered a dangerous lob from Neil Harris as Millwall at last began take the game to the home side.
Hearn then became the fourth Millwall player to be booked for a foul on Youngs as Cambridge made a final assault to snatch a winner.
And two minutes from time Warner saved the visitors again with a good save to deny Tudor.