Paul Ifill got Millwall out of jail with a late winner to register a 3-2 victory in the third-round replay over battling Cambridge.
Millwall twice came from behind against a side 43 places below them in the Football League, who had earned a 1-1 draw at the Abbey Stadium 10 days ago.
The match exploded into life with three goals in four second-half minutes after Cambridge had frustrated dominant Millwall for the best part of an hour.
First Dave Kitson put the visitors in front after 58 minutes, then Steve Claridge equalised from the penalty spot shortly afterwards, before Tom Youngs put Cambridge back in the lead.
Paul Robinson pulled the home side level for the second time with his first senior goal in his second start after 70 minutes, exactly a week after his 21st birthday.
And Ifill hit his second goal in successive games with 13 minutes left to complete a stunning comeback and set up a fourth-round clash with Southampton.
There were chances at either end in the dying moments as the visitors showed real character to fight back, but they could not quite force an equaliser.