Chris Hughton's side were well beaten by a Derby team, who once again showed they can match the best in the Championship.
A first-half Rob Hulse header was followed by a Kris Commons penalty before Shaun Barker put the result beyond doubt in the 63rd minute.
It sent a Newcastle team - unbeaten in 15 games - the same way as Nottingham Forest last month when Derby's bitter rivals also arrived as the division's form side.
The formbook clearly means little to Derby, who pushed Newcastle back from the start and produced one of the few moments of quality in a drab first half to take a 40th minute lead.
Robbie Savage, who was fortunate not to be sent off for a crude tackle on Wayne Routledge, slid a ball down the right for Commons to send in a cross which Hulse dispatched with a thumping back post header.
Routledge might have equalised within minutes only for Stephen Bywater to block his low drive and Newcastle emerged for the second half bristling with intent.
Derby were struggling to stem the tide of yellow shirts but they broke away in the 58th minute and Jonas Gutierrez tripped Michael Tonge as he surged into the area.
Commons sent Steve Harper the wrong way with a coolly taken spot kick and the 2,400 Newcastle fans knew they were facing a miserable journey back home when Derby scored again seven minutes later.
A defence which had conceded only 17 goals before the game failed to clear a corner and Hulse's overhead kick fell to Barker, who drove the all low into the bottom left corner.
Hughton responded by making a triple substitution, but it could not save Newcastle from their first Championship defeat since October and their heaviest of the season.