Heading into the 2015-16 season  Barcelona looked unbeatable from head to toe: elite managing  elite defence  elite midfield and an all-time classic trio of forwards However  the air of invincibility was sucked out in late September when Celta Vigo pulled off one of the most incredible upsets in recent memory  crushing Barcelona at Balaidos in a 4-1 romp – their worst defeat in any competition in years