Southampton were held to a 2-2 draw by Blackpool and nine-man West Ham lost 3-0 at Reading as the leading Championship promotion chasers stumbled in the race to reach the Premier League on Saturday.
Leaders Southampton were looking for their 22nd successive home win, but a shocking mistake from Polish goalkeeper Bartosz Bialkowski allowed Blackpool to become the first team to take a point from St Mary's in 2011.
Bialkowski, starting his first league match in two years after regular number one Kelvin Davis was ruled out with a groin problem, looked nervous from the outset.
Southampton striker Lambert scored on the half-hour before Chris Basham headed an equaliser before half-time.
Bialkowski then gifted the visitors a second when he let Callum McManaman's weak shot dribble through his legs and into the net.
But his blushes were in part spared by Lambert, who struck in five minutes of stoppage time to leave Saints three points clear at the top.
Second placed West Ham fared even worse as Sam Allardyce's side were crushed at the Madejski Stadium.
The Hammers, who have now lost back-to-back matches after their home defeat to Burnley last weekend, had defender Joey O'Brien sent off moments before Reading's first goal.
O'Brien received the first red card of his career for a second yellow card after a trip on Jobi McAnuff.
Alex Pearce took advantage as he drilled in the resulting free-kick in the 66th minute and substitute Simon Church tapped home Adam Le Fondre's chip in the 80th minute before Jack Collison was dismissed for a wild challenge on Jimmy Kebe late on.
Church then nodded home Ian Harte's set-piece with four minutes left to round off the scoring.
Third placed Cardiff, two points behind West Ham, failed to close the gap on the top two after struggling to a goalless draw at Millwall.
Middlesbrough are level on points with Cardiff after Scott McDonald's goal secured a 1-0 win over Brighton at the Riverside Stadium.
Hull moved up to fifth as a goal from former Manchester United midfielder Corry Evans earned a 1-0 win at bottom of the table Coventry.
Leeds retain the final play-off spot after Robert Snodgrass converted a penalty deep into stoppage time to cancel out Michael Kightly's opener.
Marlon King's brace helped Birmingham come from behind to secure a 2-1 win over Doncaster, who had taken a 50th-minute lead through Marc-Antoine Fortune.
David Norris' stoppage-time goal gave Portsmouth a 1-0 win at Burnley, while Glenn Murray netted the only goal as Crystal Palace secured an away win at Nottingham Forest.
Goals from Craig Bryson and Callum Ball helped Derby win 2-1 against Bristol City, who had taken the lead through Martyn Woolford.
Meanwhile, Paul Gallagher's opener for Leicester was cancelled out by Lee Tomlin as Peterborough secured a 1-1 draw.