Premier League leaders Chelsea surrendered a two-goal lead to lose 4-2 at home to third tier Bradford City while Manchester City lost 2-0 at home to Middlesbrough on a day of seismic FA Cup shocks.
Goals from Gary Cahill and Ramires had Chelsea cruising after 38 minutes of the fourth-round tie at Stamford Bridge but John Stead pulled a goal back before the break and former Chelsea player Filipe Morais equalised.
Andy Halliday made it 3-2 before Mark Yeates' late goal sent the Bradford fans into raptures.
Manchester City could have few complaints as they were outplayed by Championship (second tier) promotion hopefuls Middlesbrough for whom Chelsea loanee Patrick Bamford scored the opener and Spaniard Kike wrapped it up.
The surprises continued as Tottenham Hotspur lost 2-1 at home to Leicester City while in another all-Premier League tussle, third placed Southampton lost 3-2 at home to Crystal Palace.
Top-flight Swansea City went down 3-1 at second tier Blackburn Rovers, the visitors ending the match with nine men.