It had already been the club's worst run of league form since 1912, then Forest's second-string XI arrived in west London and blitzed Chelsea.
Two goals by Taiwo Awoniyi – the first after 98 seconds – and a penalty from Igor Jesus took Vitor Pereira's side six points clear of the relegation zone.
The Blues' five-game scoring drought had almost reached six by the time Joao Pedro's stoppage-time overhead kick gave them their first league goal since March 4.
Cole Palmer had a first-half penalty saved by stand-in goalkeeper Matz Sels.
Visiting supporters in the Shed End could have been forgiven for barely recognising their team. The seven outfield players brought in by Pereira, a move made with Thursday's Europa League semi-final second leg in mind, had 11 league starts this season between them.
But it was Chelsea who appeared like strangers to one another.
Interim head coach Calum McFarlane had his options reduced by late injuries to Pedro Neto and Alejandro Garnacho, leading to a first Premier League start for 18-year-old Jesse Derry.
However, the teenager required oxygen and was carried off on a stretcher with a head injury late in the first half after colliding with Forest's Zach Abbott.
The club later said the 18-year-old forward was conscious and talking while undergoing checks.
Chelsea conceded after 90 seconds when they lost possession enabling Dilane Bakwa to cross for Awoniyi, unmarked six yards out, to nod it home.
They went two down after 15 minutes. From another Bakwa cross Malo Gusto had a clear hold of Awoniyi's shirt. Anthony Taylor was called pitchside and awarded a penalty from which Jesus went straight down the middle.
Then came Derry's injury, with a penalty awarded after he and Abbott had collided while attacking a loose ball following a corner.
Abbott left the field unassisted but Chelsea's teenage debutant required lengthy treatment on the pitch.
When played resumed almost 10 minutes later, Palmer's penalty was saved by Sels.
Derry's departure meant a first appearance of the season for Levi Colwill, back after suffering an anterior cruciate ligament injury in August.
But this was a rare shaft of light for Chelsea who also lost goalkeeper Robert Sanchez, forced off with a bandage round his bleeding head following a collision with Morgan Gibbs-White.
In beween Awoniyi converted Gibbs-White's cross, narrowly staying onside to tap in from two yards out.
Pedro's stunning finish at the end drew ironic cheers from the dwindling number of Chelsea fans who had stayed.
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