Holders Barcelona suffered their first Champions League defeat in 10 matches on Tuesday when Aleksandr Ryazantsev and Gokdeniz Karadeniz netted to give Rubin Kazan a shock 2-1 win in their Group F clash.
The victory for the Russian champions, the first time Barca have lost a competitive game at home since a La Liga defeat to Osasuna last May when they had already won the Spanish title, leaves the group wide open at the halfway stage.
Inter Milan slipped to last place on three points after a 2-2 home draw with Dynamo Kiev and Barca, Rubin and Dynamo all have four points from their three games.
Barca had plenty of possession after falling behind but found it tough to break down the visitors' well-organised defence until Xavi picked out striker Zlatan Ibrahimovic with a lofted pass in the 48th minute.
Guardiola threw on midfielder Seydou Keita in place of central defender Rafael Marquez with 10 minutes left and Barca laid siege to the Rubin goal but were unable to find a second equaliser.
The final whistle prompted wild celebrations among the small knot of Rubin fans in the vast arena as the shell-shocked Barca faithfuls slipped quickly away.
Barca next face a tricky away match against Rubin in Kazan before hosting Inter. They travel to Kiev to face Dynamo on December 9.
Liverpool thrown to the Lyons
Liverpool slumped to a 2-1 defeat to Lyon in the Champions League on Tuesday to lose four straight games for the first time in 22 years.
Yossi Benayoun had put the five-time champions ahead four minutes before halftime but an injury-hit home side conceded to Maxime Gonalons midway through the second half and was then hit by a counterattack in injury time.
Liverpool also lost captain Steven Gerrard to injury and could be without the influential England midfielder for Sunday's Premier League match against Manchester United. Liverpool has three points from three Group E matches, six fewer than Lyon. The loss heaps more pressure on manager Rafael Benitez.