An injury-time goal was decisive for the second Blades game running.
A total of 3,088 Unitedites in capital as Mark Duffy gave the Blades a first half lead but Patrick Bauer's injury-time header rescued a point for Charlton.
Duffy and Clarke returned to the starting line-up for the clash at The Valley. They took the places of Scougall and Done, who dropped to the bench. The action was just ten seconds old when the home fans staged their latest protest against the Charlton Board and it took three minutes to clear the pitch of mini footballs.
The first serious goalmouth entry saw Jackson's cross evade Magennis, who, moments later slotted into the Blades net but he was flagged offside following Fox's pass.
United failed to find the early rhythm of previous fixtures but both Basham and Lafferty whipped in crosses and O'Connell twice went close in the same move from a corner by Coutts.
A clearance by goalkeeper Phillips hit Clarke but dribbled across the box but the Blades were soon ahead, following a foul by Bauer on Clarke just after the half hour. DUFFY ran over the ball and then found space from Fleck's pass to beat Phillips from an acute angle. The goalkeeper got a touch to the ball but couldn't keep it out and a glance to the dug out revealed assistant boss Knill taking the acclaim, obviously the set-piece was his influence on the training ground.
Despite a dearth of chances in the closing minutes of the first half - Ajose clipped a half-volley over Moore's crossbar - the remainder of the half saw United on top and with the last kick before the break Sharp shot at Phillips after a move involving Duffy and Clarke.
Freeman replaced Wright at the break with Basham moving into a more central position. United started the second half well but Phillips made two saves, firstly from Sharp's weak overhead kick and then from Fleck's drive.
United should have doubled the lead on 55 minutes when a space opened up for Basham to race through the Charlton defence but he was off target with the subsequent shot. A tremendous save from Phillips then prevented the home side from falling further behind, turning away a Duffy shot after he linked up with O'Connell and Coutts.
Lavery replaced Clarke and soon after Scougall took over from Duffy, whilst the chances dried up until the final ten minutes. From a Coutts free-kick, Lavery's header was saved and then gathered, whilst a minute later Moore kept out Magennis.
United enjoyed a lot of possession late on but could not add a second and four minutes of added time gave Charlton hope, which they took advantage of. Lafferty's foul on Botaka gave the home side a free-kick and from Chicksen's inswinger, BAUER headed in from close range.
For the final moments Charlton were on top and a corner and long throw were repelled before the final whistle.
Charlton: Phillips, Foley, Jackson (Chicksen 52), Bauer, Lookman, Crofts (Botaka 81), Magennis, Ajose (Novak 63), Pearce, Ulvestad, Fox.
Unused: Konsa, Lennon, Hanlan, Beeney.
Blades: Moore, Fleck, O'Connell, Basham, Sharp, Wright (Freeman 45), Coutts, Ebanks-Landell, Duffy (Scougall 75), Lafferty, Clarke (Lavery 66).
Unused: Hussey, Done, Ramsdale, Chapman.