The two teams were then drawn to play each other in the Championship play-off semi-finals, with a lot of the build-up to the game dominated by "Spygate".
It turned out that the man was a Leeds employee.
And while Derby had indeed failed to beat Leeds all season, Lampard's first win against Marcelo Bielsa's team gave him the flawless opportunity to celebrate by singing his own song.
Leeds went into the second leg of this play-off semi-final with a one goal advantage thanks to Kemar Roofe's victor at Pride Park, but Leeds being Leeds, they crumbled when it mattered most, allowing Derby back into the game.
They then took an early lead at Elland Road to make them nearly certain to progress to the final.
But a period in the ascendancy for Bielsa's side ended when Gaetano Berardi received a second yellow card for a rash challenge on Bradley Johnson in the 78th minute.
Jack Marriott came off the bench to fire in two goals as Derby secured a 4-3 aggregate win over their rivals to set up a match against Aston Villa to determine which team will take the final place in next season's Premier League.
During the on-pitch celebrations, Derby's players could be seen miming the wearing of binoculars in front of their fans. The Chelsea loanees Mason Mount and Fikayo Tomori posted the same message on Twitter: "I spy with my little eye, something beginning with W".
Lampard also admitted that the victory was made sweeter by the "Spygate" controversy from earlier this season. "I'm not gonna lie maybe it is", he told Sky Sports, per the Mirror.
"I said to the lads that everyone had written us off". "Sometimes feelings can be wrong but when we got the first goal back the belief just began to rise", he said.
"There's work to do, we're underdogs again".
But Bielsa had no complaints about his players throughout the campaign, saying: "What they have offered to the club during this season will be valued with time".
"We'll have a tough game a Wembley against a brilliant Villa team, but we're there".