Rhys Norrington-Davies' horror own goal sent Fleetwood into the last 16 of the Carabao Cup for the first time as the League Two side stunned Championship Sheffield United 1-0 at Highbury.
Norrington-Davies nodded a Danny Andrew cross into his own net from close range in the 57th minute to earn Fleetwood a .
Matt Lawlor's side, unbeaten now in nine games in all competitions, had another Andrew effort ruled out for offside and twice hit the woodwork in a dominant second-half display.
Tom Cannon did put the ball in the net for Chris Wilder's visitors, who lined up with only four starting survivors from last Friday's 1-0 league defeat by Wolves, but an offside flag was raised before the striker slid the ball into the net.
United had started positively and Ollie Arblaster had a goalbound effort blocked after only 36 seconds.
But Mark Helm forced Blades goalkeeper Cooper into a seventh-minute save before former United star Ched Evans put his follow-up header narrowly wide.
Jay Lynch denied Joe Rothwell as the visitors attacked again but Cooper was the busier, saving from Lewis McCann and Crispin McLean.
Helm's dipping right-foot effort from 18 yards grazed the top netting as Fleetwood finished the first half in the ascendancy, and they deserved their break from Norrington-Davies' own goal.
United finally came to life in the last 10 minutes with Lynch brilliantly denying Sydie Peck and Ben Brereton Diaz while Cannon saw a goalbound strike superbly blocked.
But the equaliser proved elusive as the away fans made their displeasure known at the whistle.
