Barcelona fell victim to a huge Champions League shock in Glasgow, losing 2-1 to Celtic at Parkhead.
The Blaugrana arrived in Scotland with a 100 per cent record in Group G and were intent on securing last 16 qualification early, but perhaps surprisingly left Cesc Fabregas and David Villa on the bench.
Youngster Marc Bartra started in central defence and Alex Song appeared in his preferred midfield position. The hosts Celtic were missing Gary Hooper and Emilio Izaguirre but Georgios Samaras – who scored at Camp Nou – won his race to be fit.
The game started like the last meeting between the two ended, with Celtic hemmed in and Barca dominating possession, but the Hoops took a shock lead against the run of play in the 21st minute!
Again a setpiece was Barcelona's downfall, Victor Wanyama netting a powerful back post header from Charlie Mulgrew's corner.
Barca were the width of a crossbar away from equalising when Andres Iniesta fed Lionel Messi, the Argentine rattling the woodwork with a rising left foot shot.
Alexis Sanchez then nodded a Pedro cross back across goal but it flicked off the post and wide.
After a slow start to the second half by the Catalans Sanchez got in behind and forced a double save from Celtic 'keeper Fraser Forster.
Song was lucky not be red carded in the 67th minute, taking Miku's legs away from him when he was already on a yellow, and was promptly replaced by Fabregas.
Messi played a quick one two with Jordi Alba then shot but Forster produced another flying save.
Barca were piling on the pressure looking for an equaliser but astonishingly it was Celtic who scored again! Xavi misjudged a long punt from Forster and substitute Tony Watt – aged just 18 – lashed a low finish past Victor Valdes.
Messi gave the visitors hope in the 91st minute, tucking in a rebound after Forster blocked Pedro's low effort, but Celtic held on for a historic victory on their 125th anniversary.
It was only Barca's second loss of the season in all competitions, and their first in the Champions League group stage for over three years.
Celtic 2 Barcelona 1
Scorers: Wanyama 21 (C) , Watt 82 (C) Messi 91 (B)
Celtic: Forster; Lustig (Watt 71), Ambrose, Wilson, Matthews; Commons, Wanyama, Mulgrew, Ledley; Samaras (Kayal 79); Miku
Barcelona: Valdes, Alba, Bartra (Pique 71), Mascherano, Alba; Song (Fabregas 71), Xavi, Iniesta; Messi, Sanchez (Villa 64), Pedro
Ref: Kuipers (NED)