Striker David Nugent, the man with one England cap to his name, slotted home the winner to take Leicester into the quarter-finals of the FA Cup in a stirring encounter with Norwich at Carrow Road.
Leicester were good value for their victory as they demonstrated there is a fine line in talent between the middle of the Premier League and the middle of the championship.
Sean St Ledger put Leicester in front in the first half but Norwich captain Wes Hoolahan equalised when he stabbed home the rebound after Leicester goalkeeper Kasper Schmeichel had saved his spot kick.
Leicester's victory might have been more convincing if Wes Morgan's backheel, which appeared at first sight to have crossed the line, had been allowed.
Jermaine Beckford also hit the bar as Norwich's high-flying season took an unexpected jolt.
Lambert's side lie in eighth place in the Premier League with their survival already virtually guaranteed and such is the confidence in the squad that Lambert barely considers fielding weakened teams in the cup.
There were four changes to the starting line-up from the side which beat Swansea away last weekend.
Lambert rested leading scorer Grant Holt and first-choice goalkeeper John Ruddy was on the bench, allowing his understudy Jed Steer a piece of the action.
Captain Hoolahan returned, along with Leon Barnett, and striker James Vaughan started on the bench after five months sidelined with a knee injury.
It was Leicester, however, who took the game to Norwich in the first half. Nigel Pearson's side were intent on an attacking game and they took a shock lead after five minutes.
It came after a series of three corners whipped in by Ben Marshall. The first two were scrambled away by the Norwich defence but the third was met crisply by the head of St Ledger who guided the ball past goalkeeper Steer and into the net.
It was deserved reward for Leicester's enterprise and for Marshall, in particular, who was causing Norwich all sorts of problems
Norwich were back on terms after 22 minutes, however, when Leicester goalkeeper Schmeichel had an aberration, barging into Elliott Bennett in the penalty area as the ball spun towards them.
Referee Mike Dean pointed to the spot and Hoolahan stepped up.
Schmeichel almost made amends for his error, pushing away Hoolahan's spot-kick but only as far as the Norwich captain who coolly slotted home the rebound.
Norwich were back on terms but Leicester, with Richie Wellens industrious in midfield, were still giving as good as they got and on the half-hour their players were claiming they should have gone ahead from a corner.
The ball landed at the feet of defender Morgan who backheeled the ball goalwards only to see it strike the chest of Norwich defender Elliott Ward, who appeared to have both feet behind the goal-line.
Leicester claimed the ball had crossed the line but Mr Dean and his assistant waved play on. On first sight it looked a harsh decision but television replays struggled to show all of the ball had crossed the line - another case where goal-line technology would have settled matters.
There was no let-up in the attacking intent of both sides in the second half. The last thing either side wanted was a replay and Norwich should have gone ahead after 53 minutes but Bennett saw his goalbound shot hooked off the line by Wellens.
On the hour mark, Lambert made a double change, Vaughan coming on for Simeon Jackson and Aaron Wilbraham for Morison.
It was Leicester, however, who stepped up the energy levels and it was just reward when they went ahead after 70 minutes.
Beckford flicked the ball on and Nugent stepped inside defender Barnett far too easily before slotting a precise right-foot shot past Steer.
Norwich launched a late rally and brought two fine saves from Schmeichel but it was Leicester who went closest to scoring again when Beckford hit the crossbar with a left-foot shot.