After an entertaining encounter and with the game inevitably looking like extra-time, both sides were launching late attacks in search of a winner.
With the fixture wide open, it was finally the Championship side who stole it with a winner in the third minute of stoppage time.
The ball dropped for Smith inside the area and he fired past Romero for Ashton Gate to go wild. At full-time, a few hundred fans invaded the pitch in celebration of what is arguably the most important goal in Bristol City's history.
Carabao Cup: Bristol City vs Manchester United | Smith (GOAL) 93' pic.twitter.com/yEpihoZNj0
— CY Enterprise (@CY_Enterprise) 20 December 2017