Walsall confirmed their status as Sheffield United's bogey team when they beat Chris Wilder's for the third time this season.
Goals from Amadou Bakayoko, Jason McCarthy, Joe Edwards and an Erhun Oztumer penalty saw the hosts claim a win which, despite their previous successes over United, still ranked as a surprise.
The visitors, who remain top of the table with 19 matches left to play, saw Jack O'Connell cancel-out Bakayoko's opener during a thoroughly absorbing first-half. The second, whilst also entertaining, nevertheless made miserable viewing the United manager Chris Wilder as the opposition netted three times without reply.
Although the scoreline exaggerated the difference between the two sides, defeat does not sit comfortably with a squad which had arrived in the Midlands hoping to record a seventh straight victory.
Walsall might not look like troubling the top six this season but, as the League One leaders discovered, still possess some prodigiously talented footballers. Oztumer's touch and industry caused problems while Isiah Osbourne made his physical presence felt. Another, Bakayoko, was responsible for edging Jon Whitney's men in front before O'Connell, who later appeared fortunate to escape a red card, equalised.
McCarthy restored Walsall's advantage and, as United chased their second of the afternoon, Edwards duly nudged them further behind. Oztumer shattered any hopes of a late comeback when he converted a 76th minute penalty following a defensive mix-up.
Bakayoko had scored the only goal of the game when these two teams met at Bramall Lane earlier this season.
There is something about United which clearly brings out the best in him and, with only four minutes on the clock, the 21-year-old from Sierrra Leone pounced again. But the self-belief Wilder has injected into his squad was evident when, with the home crowd still celebrating Walsall's breakthrough, O'Connell dragged them back on level terms. Rather than panic, United simply dusted themselves down and responded in the best possible fashion with a scrambled but thoroughly deserved effort.
But Wilder and his coaching staff will have serious reservations about the manner in which the visitors conceded a second goal just before the hour. Jason McCarthy's low shot to the near post was well-placed but both Ethan Ebanks-Landell and Simon Moore appeared disappointed with their failure to snuff the opening out.
When Edwards pounced, United were staring down the barrel of a gun. Oztumer was responsible for pulling the trigger when he claimed Walsall's fourth from the spot.
With Matt Done deputising for the injured Daniel Lafferty at wing-back, Walsall predictably looked to test the centre-forward's defensive capabilities. But Done, who coped well with the barrage of diagonal balls swept towards the corner flag during the early exchanges, made his presence felt at the opposite end of the pitch too by setting in motion the move which ended with Chris Basham testing Neil Etheridge's handling.
When Billy Sharp saw a first-time effort rebound back off the crossbar, Whitney must have feared the worst. But seconds later, darting into the space vacated by Kieron Freeman who had teed--up his captain, Bakayoko edged the hosts in front with a low drive across Moore.
O'Connell, though, quickly restored parity when Ebanks-Landell turned Mark Duffy's 11th minute set-piece, following a foul on Caolan Lavery, back across the six yard box. It was the centre-half's second goal in as many games. Lavery went close soon after, trading chances with former United midfielder Florent Cuvelier who sent a long-range shot flying inches over Moore's bar.
Moore's ability to produce big saves at key moments has contributed to United's success so far this term. Almost immediately after the restart, he denied Bakayoko a second goal with an excellent one-handed stop.
But, in the 58th minute, Moore was unable to prevent McCarthy's shot sneaking across the line before conceding another, this time from Edwards, 10 minutes later. In between, O'Connell seemed to handle another Edwards attempt. Walsall, having already beaten United in both the league and Checkatrade Trophy earlier this term, made sure they completed the unlikeliest of hat-tricks when Oztumer claimed their fourth of the contest.
Walsall: Etheridge, Edwards, O'Connor, McCarthy, Chambers, Cuvelier (Dobson 75), Oztumer (Morris 88), Osbourne, Preston, Bakayoko (Jackson 78), E O'Connell. Not used: MacGillivray, Laird, Dobson, Jackson, Roberts, Makris.
Sheffield United:L Moore, Fleck, J O'Connell, Basham (Clarke 68), Lavery (Scougall 78), Sharp, Done, Coutts, Freeman, Ebanks-Landell, Duffy. Not used: Ramsdale, Hussey, McNulty, Wright, Wilson.
Referee: Richard Clark (Hampshire). Attendance: 6,899