PETERBOROUGH United deserved their victory but, for a second time in 12 months, Derby County left London Road reflecting on what might have been.
Last season, the Rams built a two-goal lead and missed a glorious chance to make it three before Posh stormed back to win 3-2.
Saturday's 3-0 scoreline looks more emphatic but tells only part of the story of what was a strange contest containing a number of turning points.
Fall the wrong side of turning points and defeat usually follows.
For half an hour in the first half and a period in the second, Derby harboured genuine hope of stretching their unbeaten run to six matches.
They failed to do so because their cutting edge was blunt, certain players had off days and silly mistakes at the back proved costly.
Having to play for the final 20 minutes with 10 men following Jake Buxton's red card also hindered them. They trailed by only one goal at the time before Saido Berahino's late double added gloss to the day for Peterborough, who climbed out of the bottom three for the first time this season.
Rams manager Nigel Clough had warned that Peterborough, now with four wins in six games, can beat any team on their day.
This was such a day but definitely not Derby's day.
Their level of performance slipped when compared with what we have seen for much of the season. Not by much but enough to send them to their heaviest defeat since they lost by four at Southampton in February.
Yet there was no sign of such an outcome earlier in the afternoon.
Much of Peterborough's play tends to go through their deep-lying central midfielder Grant McCann. Once Derby got close to him and restricted his influence, they enjoyed a modicum of control and looked relatively comfortable.
Richard Keogh headed wide from one of a number of corners won in the opening 15 minutes and Theo Robinson whipped a shot from distance narrowly wide.
Conor Sammon's unselfish running and work stretched Peterborough's defence, although it soon became evident that Craig Bryson's energy and willingness to get forward would be missed.
Bryson was out with a tight hamsting and his place in the middle of midfield went to Jeff Hendrick, who played alongside Will Hughes in a 4-4-2 formation.
Little had been seen of Peterborough in attack before they went ahead after 30 minutes.
Shaun Brisley's header from McCann's corner was blocked on the line and Hughes managed to hack clear only for Michael Bostwick to smash a spectacular volley from 20 yards into the top right-hand corner of the net.
Goals change games and Peterborough suddenly looked a different side.
They might have scored twice more within minutes but were denied by Adam Legzdins, who parried George Boyd's low drive and brilliantly tipped Berahino's shot on to the bar.
Storm weathered, there was time in the half for Derby to show they were still in it.
Sammon had two sights of goal without producing a finish and Michael Jacobs fired straight at Bobby Olejnik after his free kick had been blocked.
Derby's best chance to equalise arrived five minutes after the break when a slick move sliced Peterborough open. Sammon's vision released John Brayford on an overlap and his low centre was met by Robinson six yards out.
The striker has been nicking goals from close range this season but, on this occasion, his first-time shot smacked against the outside of a post.
A golden opportunity and a turning point, no question, because the Rams were on top.
They still needed the impressive Legzdins to shine and he saved Lee Tomlin's 56th-minute penalty after Keogh was judged to have brought down Boyd.
Mistakes by Keogh have been rare since he signed from Coventry City. His slip allowed Boyd to charge into the area and the defender was asking for trouble once he went to ground in a bid to retrieve the situation.
He was adamant he got a toe on the ball and it looked a soft decision but it is easy to see why a spot kick was awarded, especially as he dived in, because he was always likely to take Boyd as well.
Derby, grateful for the let-off, pressed again before the next big decision by referee Simon Hooper provided another turning point.
Buxton had not long been booked for a foul on Berahino when he was shown a second yellow card following a tackle on Boyd to leave Derby down to 10.
The Rams defender, not one for demonstrative reactions, argued that he got the ball and he looked to have a case.
Peterborough boss Darren Ferguson felt the minutes between his side failing from the penalty spot and Buxton's sending off were crucial.
"Derby received a lift from the penalty miss but we didn't let our heads go down and kept our concentration when they pushed us back," said Ferguson.
"The red card killed them, to be fair, and we could see out a big win against a very good side."
Legzdins saved from Bostwick and substitute Danny Swanson and the home side should have had another penalty when Paul Coutts sent wing-back Mark Little tumbling.
A second Peterborough goal five minutes before the end of normal time finally settled matters.
Legzdins will not want to see a replay as he let a tame shot from Berhino slip through his hands and the ball bobbled over the line. It was a cruel and undeserved moment for the goalkeeper because he had done much to keep Derby in the game, just as he did at Ipswich a few days earlier.
Berahino, a 19-year-old on loan from West Brom, capped his lively display in the fifth minute of added time when he raced clear of an offside trap and coolly beat Legzdins.
![Key moments go against Derby County in 3-0 defeat to Peterborough United Key moments go against Derby County in 3-0 defeat to Peterborough United]()