Two defensive errors handed Crawley Town victory on their first ever visit to Bramall Lane.
Neill Collins and Harry Maguire were responsible for the lapses in concentration as the Blades failed to make ground in the promotion race.
The Blades are now without a home with in four matches, despite dominating the opening 75 minutes of the encounter, with Ryan Flynn and Michael Doyle both striking posts.
Boss Wilson made just one change from the weekend draw, recalling the fit-again Flynn at the expense of fellow Scot Robson, who dropped to the bench. Crawley's team included ex-Blades loanee, Clarke.
United were forced to kick towards the Kop in the first period but not before a pre-match huddle. Flynn and Murphy took up wide positions and the latter swung in an early free-kick as plenty of noise was offered from the stands in the opening exchanges.
Flynn and Murphy were positive in taking on their markers in the first ten minutes and a Doyle effort was blocked as Crawley retreated. Both sides should have done better with headers but Jones failed to get a significant touch on Sadler's cross and Collins directed a Flynn corner wide.
Proctor and Torres both had shots for Crawley, whilst for United long-range efforts from McDonald and Higginbotham, from a free-kick, failed to find the target.
The best attempts from both sides in the first half arrived in the final five minutes. On 41 minutes, Flynn beat Sadler on the outside and his shot across goal struck the base of a post and hit Kitson on the away and, moments later at the other end, Clarke met a cross from Adams two yards out but the Crawley contingent were amazed to see the big hand of Long clawing the ball away to safety.
Three minutes into the second half the Blades struck the woodwork again. This time it was a 20-yard volley from Doyle that came back off a post and Flynn was unable to control the rebound.
Crawley hit back with Torres firing at Long, despite having space in the box, Clarke and Adams fired over, whilst there was a penalty appeal when the ball struck Kitson but referee Miller said no.
Wilson made a double change, Murphy and Forte replaced by McFadzean and Porter, the latter winning a free-kick in a dangerous position within moments but Doyle's free-kick struck the wall. On 74 minutes Kitson dug out a cross which Porter met but the subsequent header was just wide.
Disaster struck, however, on 77 minutes when Collins was caught in possession. Hayes progressed down the right but intelligently found CLARKE in space and the former Lane loanee slid the ball under the body of the advancing Long.
In the final ten minutes a Maguire free-kick popped out of the wall but Porter was unable to take advantage, whilst Higginbotham was unable to get a significant touch to Flynn's corner.
Robson replaced Flynn but there was further heartache for Blades fans when Maguire lost the ball under his feet and HAYES took advantage, another Crawley player coolly slotting past Long.
Late on Porter had a shot pushed around the post by goalkeeper Jones and four minutes of added time was unable to save the Blades, who continued to huff without any reward.
Blades: Long, Higginbotham, Maguire, Hill, Flynn, Doyle (Robson 83), McDonald, Kitson, Collins, Forte (Porter 66), Murphy (McFadzean 66).
Unused: Westlake, Ironside, Kennedy, Coyne.
Crawley: P. Jones, Sadler, Essam, Clarke (Connolly 86), Torres (O'Brien 88), Walsh, Hunt, Proctor (Hayes 70), Bulman, Adams, M. Jones.
Unused: Dumbuya, Elford-Alliyu, Maddison, Malins.