Lincoln City produced a huge FA Cup shock to beat top-flight Burnley 1-0 away and become the first minor league club to reach the quarter-finals for 103 years on Saturday.
On a day of fifth-round toil for the top-flight sides, English champions Leicester City's problems continued to mount as they went down 1-0 at third-tier Millwall, who played most of the second half with 10 men.
Chelsea, chasing a league and FA Cup double, restored some order with goals from Pedro and Diego Costa earning them a 2-0 win at Championship side Wolverhampton Wanderers, but Manchester City were held to a 0-0 draw at Huddersfield Town.
Middlesbrough quelled a stirring comeback by third-tier Oxford United to win 3-2 at the Riverside Stadium.
Millwall's third win over Premier League opposition this season, secured by Shaun Cummings's 90th-minute goal at the New Den, would ordinarily have topped the shocks.
But they were outdone by Lincoln.
Sean Raggett's headed goal in the 90th minute continued the dream run of the National League leaders who easily matched a side 81 places higher up the English football ladder and who held Premier League leaders Chelsea to a draw a week ago.
Burnley had lost only three of their previous 29 home matches in all competitions but failed to make their perceived superiority show in a scrappy game.
Just when it seemed Burnley had gained the upper hand Lincoln were awarded a corner and Raggett rose above a crowd of players to head the ball over the line despite keeper Tom Heaton's attempt to claw it out.
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Lincoln's Sean Raggett scores their winning goal against Burnley Reuters
Lincoln, only the third minor league side to beat top-flight opposition in nearly 30 years, are now just one win away from a semi-final at Wembley Stadium.
Leicester manager Claudio Ranieri made 10 changes for his side's trip to south London Millwall -- his eyes fixed on the his side's relegation battle and a looming Champions League last-16 tie against Spanish club Sevilla.
Millwall had Jake Cooper sent off in the 52nd minute for a second booking but even a numerical advantage failed to stir a Leicester side who suffered a sucker punch when Cummings was played in by Lee Gregory and fired past Ron-Robert Zieler.
Chris Maguire and Toni Martinez scored within a minute of each other to haul Oxford level at Middlesbrough but their hopes of emulating Lincoln and Millwall were ended four minutes from time when Cristhian Stuani tapped in Boro's winner.
Promotion-chasing Huddersfield enjoyed a record crowd of 24,000 at their John Smith's Stadium as they matched Guardiola's City. "A clean sheet against Manchester City makes our chest bigger," Huddersfield boss David Wagner said.
Burnley 0 Lincoln 1
Huddersfield 0 Man City 0
(Leadbitter 26-pen, Gestede 34, Stuani 86) Oxford 2
(Maguire 64, Martinez 65)
(Cummings 90) Leicester 0
Wolves 0 Chelsea 2
(Pedro 65, Costa 89)