It had looked like Lewis McGugan's 88th-minute penalty would salvage a draw for Forest but Chambers deflected Jem Karacan's cross past Lee Camp, who then went on to save a spot-kick from Shane Long.
Ian Harte put Reading ahead but a Kris Boyd penalty and a goal from Robert Earnshaw turned the game on its head.
Karacan and Jimmy Kebe restored the Royals' lead before McGugan levelled from the penalty spot, shortly before Chambers' unfortunate intervention.
Source: PA
Source: PA