Full-time: Buxton off as Rams go down 3-0 at Peterborough
DERBY County's five-game unbeaten run was ended by a 3-0 defeat against Peterborough United at London Road.
Midfielder Michael Bostwick gave Posh the lead after half an hour.
Rams keeper Adam Legzdins saved Lee Tomlin's second-half penalty before Jake Buxton was sent off for a second bookable offence.
Legzdins had kept Derby in the game with a string of saves but his error allowed Saido Berahino to add a second goal in the 85th minute.
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And Berahino scored again five minutes into stoppage time to compound the visitors' misery.
Derby boss Nigel Clough made two changes to his side following the 2-1 midweek win at Ipswich.
Craig Bryson missed out due to a tight hamstring but Jeff Hendrick (ankle) was fit to return in central midfield.
At left-back, Gareth Roberts was recalled after recovering from a hamstring injury, so James O'Connor dropped to the bench.
The Rams lined up in a 4-4-2 formation and started brightly.
In the fifth minute, Richard Keogh rose the highest to meet Paul Coutts' corner but could not direct his header on target.
Derby looked a threat going forward and Theo Robinson was not too far wide with a 30-yard strike.
The home side gradually found their way into the game, with George Boyd firing straight at Legzdins and Berahino flashing an angled drive past the post.
Posh took the lead on the half-hour.
Grant McCann's corner was sent goalwards by Shaun Brisley. Will Hughes managed to clear off the line but Bostwick returned the ball with interest and his 20-yard volley flew into the net, despite the efforts of Roberts to keep it out.
Boosted by the goal, Peterborough might have had a second soon afterwards.
First, Boyd's low drive was well parried by Legzdins and the Rams keeper then did brilliantly to tip Berahino's effort on to the bar and over.
Having survived the onslaught, Derby went close to an equaliser five minutes before the break.
A neat move ended with Conor Sammon spinning sharply in the box, only to fire across the face of goal, possibly via a slight touch from goalkeeper Bobby Olejnik.
At the other end, Legzdins had to be alert to hold on to Kgosi Ntlhe's far-post header from McCann's corner.
In first-half stoppage time, Michael Jacobs fired straight at Olejnik after his free kick had rebounded off the wall.
Derby almost drew level five minutes into the second half.
Sammon's fine pass released John Brayford on the right and his low cross was met by Robinson six yards out but his first-time shot smacked against the outside of the post.
The Rams were on top but they conceded a penalty after 55 minutes.
Keogh lost possession to Boyd and as he tried to recover, the Derby skipper was adjudged to have fouled the Peterborough man in the box.
Tomlin stepped up by saw his spot kick saved by Legzdins, away to his right, and as the ball looped up in the air, Jake Buxton headed behind.
Derby pressed for an equaliser but could not find a way through, despite some heavy pressure.
Their cause was not helped when they were reduced to 10 men with 20 minutes to go.
Buxton had not long been booked for a foul on Berahino when he was shown a second yellow card following a tackle on Boyd.
The Derby defender argued that he got the ball and he looked to have a case.
Clough brought on Mark O'Brien as defensive cover, with Jacobs the man to be sacrificed.
Legzdins was called into action again in the 73rd minute to keep out Bostwick's low shot in a crowded area.
Nathan Tyson replaced Roberts and Derby were becoming more and more stretched at the back.
Mark Little had a penalty shout turned down and Danny Swanson's volley was beaten out by Legzdins before O'Brien completed the clearance.
The second goal arrived with five minutes remaining.
Berahino curled a speculative shot goalwards from 25 yards and Legzdins, whose saves had kept Derby in the match, could only watch in horror as the ball slipped through his fingers and into the net.
Legzdins had otherwise had a fine game and, in added time, he was out quickly to block Little's shot.
PETERBOROUGH UNITED: Olejnik, Alcock, Brisley, Knight-Percival, Little, Bostwick, McCann, Ntlhe, Tomlin (Sinclair, 79), Boyd (Swanson, 75), Berahino. Subs: Richardson (gk), Zakuani, Barnett, Newell, Ferdinand.
DERBY COUNTY: Legzdins, Brayford, Keogh, Buxton, Roberts (Tyson, 79), Coutts, Hendrick (O'Connor, 84), Hughes, Jacobs (O'Brien, 72), Robinson, Sammon. Subs: Deeney (gk), Freeman, Davies, Bennett.
REFEREE: S Hooper (Wiltshire).