Warnock left the Eagles in March after being offered the Loftus Road job and Adel Taarabt broke the deadlock in the 49th minute.
Palace looked to have denied the R's an eighth victory from ten games this season when Kieron Cadogan equalised in the 89th minute.
Heidar Helguson's fifth goal of the season snatched three stoppage-time points for the visitors.
Second-placed Cardiff City ground out a 2-1 victory over Barnsley at Oakwell despite playing the final minutes with 10 men.
Jason Shackell cancelled out Craig Bellamy's 42nd minute opener for the Bluebirds. Peter Whittingham missed a penalty but Seji Olofinjana earned the points for the visitors with a 67th minute winner, before Darcy Blake was dismissed.
Norwich City remain third in the table following a 3-0 victory at Bristol City. A brace from Simeon Jackson and penalty from Wes Hoolahan put the Robins to the sword at Ashton Gate.
Leeds United failed to bounce back from their 6-4 humbling at the hands of Preston in midweek as they lost 2-1 at Ipswich Town. Jason Scotland opened the scoring for the Tractor Boys only for Robert Snodgrass to restore parity midway through the second half.
Leeds shot themselves in the foot as Alex Bruce saw red and Tommy Smith crashed home an 83rd minute winner for Roy Keane's side.
Middlesbrough threw away a two-goal lead to draw 2-2 with fellow strugglers Portsmouth at the Riverside.
First-half goals from Matthew Bates and Barry Robson, via the penalty spot, had put the Teessiders in the driving seat. David Nugent halved the deficit on the hour mark and Liam Lawrence salvaged a point with an 83rd minute equaliser from the penalty spot.
Sheffield United's season continued after they went down 1-0 at Watford with Marvin Sordell's early goal separating the sides.
Doncaster Rovers and Nottingham Forest shared the spoils following a 1-1 draw and it was the same scoreline in the matches between Millwall and Burnley and Preston against Reading.
Swansea City and Derby County played out a low-key goalless draw at the Liberty Stadium, while the match between Hull City and Coventry also ended scoreless at the KC Stadium.