Promotion chasing Forest took on already relegated Brentford, who had nothing more than pride to play for, but it was the home side who made an early break through.
Lloyd Owusu played the ball to French midfielder Thomas Pinault on 15 minutes, and his chip from 20 yards struck the underside of the bar and crossed the line to leave Paul Smith helpless.
Just after the half hour Brentford could have doubled their lead when the lively Charlie Ide played in Owusu on the edge of the box, but his shot flew over the bar.
Forest's best chance of the half came just before the break when Kris Commons low cross found Lewis McGugan, but he failed to keep his shot on target.
Brentford doubled their lead just a minute after the re-start. Owusu held the ball up and played it into the path of the impressive Ide, whose left-foot drive from eight yards was too powerful for former Bees keeper Smith at the near post.
Forest gave themselves a fighting chance of snatching something from the game five minutes later when Commons' corner found David Prutton, who headed in from close range.
The comeback was nearly complete on 55 minutes when Grant Holt found himself unmarked in the centre of the goal, but Stuart Nelson in the Brentford goal pulled off a brilliant point-blank save.
The equaliser finally came through influential midfielder Commons, his shot from the edge of the area crossed the line despite the best efforts of Nelson.
On 73 minutes the turnaround was complete when Adam Griffiths brought down Jack Lester in the box and referee Richard Beeby pointed to the spot.
Holt stepped up and fired the ball down the centre giving Nelson no chance and Commons added a fourth in stoppage time to complete the comeback.