Amadou Bakayoko scored the only goal as Walsall stunned high-flying Sheffield United at Bramall Lane.
The decisive moment came on 42 minutes, when the 20-year-old striker chased Erhun Oztumer’s long pass before prodding past Simon Moore.
United had plenty of chances – the best of which fell to local lad Billy Sharp but his weak penalty was saved by Neil Etheridge.
The Blades also had two goals disallowed, as well as a host of other opportunities, but the Saddlers held on for a memorable win.
Before kick-off, both teams observed a minute’s silence in memory of the victims of the tragic air accident in Colombia.
Sheff United dominated possession in the early stages, although it was Walsall who had the game’s first real opportunity on ten minutes.
Joe Edwards was brought down by Ethan Ebanks-Landell 25 yards out, and Oztumer curled the resulting free-kick over the wall but against the bar.
United, unbeaten in their previous 15 league games, responded with Mark Duffy crossing for Leon Clarke to head wide at the far post.
The same pair combined again moments later, Duffy sliding a pass through to Clarke, who could only find the side-netting from a difficult angle.
With the Saddlers sitting deep, Chris Wilder’s Blades continued to force the initiative.
The influential Duffy steered a right-footed effort wide from distance and a John Fleck drive was blocked.
The closest the hosts came in the first half was on 33 minutes, when Jack O’Connell’s header from Fleck’s corner was hacked off the line by Joe Edwards.
Six minutes later, a low strike from Daniel Lafferty forced Etheridge into a fine stop at his near post.
Walsall had barely threatened but, against the run of play, Jon Whitney’s men took a surprise 42nd-minute lead.
Erhun Oztumer’s long, floated pass was chased by the recalled Bakayoko, who poked the ball past the onrushing Simon Moore for his seventh goal of the season.
Half-time: Sheff Utd 0-1 Walsall
The visitors almost doubled their lead within 60 seconds of the resumption, when Oztumer’s powerful, rising drive was tipped over one-handed by Moore.
United had the ball in the net on 56 minutes when O’Connell nodded home a corner but referee Darren England had already blown for a foul.
With the home fans growing increasingly frustrated, Sheff Utd began to pile on the pressure.
A 25-yard effort from Fleck sailed over before Chris Basham headed wide after Duffy’s in-swinging free-kick was missed by Etheridge.
United went even closer midway through the half, as Fleck’s inventive, lofted pass was flicked over the bar by Lafferty.
Still the home side came forward.
With 13 minutes remaining, Duffy attempted to catch out Etheridge at his near post with a clever free-kick, but the keeper was equal to it, pushing the ball behind.
Wilder’s men were presented with a golden chance to level six minutes from time.
Fleck’s drive was handled by James O’Connor inside a crowded penalty area, with the referee pointing straight to the spot.
However, Sharp’s subsequent penalty lacked sufficient power, enabling Etheridge to save comfortably to his left.
The relentless Blades refused to give up the ghost - and again they had the ball in the net as the clock ticked towards 90.
Substitute Harry Chapman’s wicked right-wing cross was bundled over the line by Sharp from close-range – only for the goal to be disallowed for handball.
Sheff Utd continued to press.
The luckless Sharp flicked a stoppage-time effort wide from a corner, but the Saddlers held on for all three points!
Full-time: Sheff Utd 0-1 Walsall
Team (3-5-2): Etheridge; O’Connor, Preston, Laird; Morris, Chambers, Edwards, Dobson (Osbourne 69’), McCarthy; Oztumer (Ginnelly 69’), Bakayoko (Jackson 77’).
Subs not used: MacGillivray, Cuvelier, Kinsella, Makris.
Attendance: 18,343 (178 Walsall supporters).