The caution given to Celtic goalkeeper Artur Boruc, who crossed himself during a match against Rangers, has brought the issue of sectarianism in Scotland back into sharp focus.
The club rivalry is historically tied up in religion
The rivalry between Glasgow clubs Rangers and Celtic - known collectively as the Old Firm - is historically tied up in religion.
Celtic were formed in 1888 by Irish Catholic immigrants who began emigrating to the West of Scotland in the 1840s and their descendants.
Rangers, who were formed in 1873, have always been perceived as "the Protestant club" and Celtic "the Catholic club".
Up until 1989, when Mo Johnston signed for them, Rangers had never fielded a high-profile Catholic player.
Yet a range of people support the clubs, including those without religion and those who belong to other religions.
The majority of Rangers and Celtic supporters do not get involved in sectarianism.
But it is the minority who tend to grab the headlines.
Old Firm matches have always been tense affairs and the mutual antipathy between the supporters has often been stirred up by sectarian songs and chants on both sides.
Paul Gascoigne caused controversy with his flute gesture
In the 1970s the chants became increasingly influenced by The Troubles in Northern Ireland, with some Celtic fans supporting the Provisional IRA and some Rangers fans applauding the actions of loyalist groups.
The tribal nature of Old Firm clashes is underlined by Rangers fans flying Union flags and Celtic followers waving the Irish tricolour.
The mutual animosity was outlined four years ago when some Celtic fans began flying Palestinian flags and some Rangers supporters responded by fluttering Israeli flags.
In 1999 leading QC Donald Findlay resigned his position as vice chairman of Rangers after he was caught on video singing a sectarian song, The Billy Boys, after the Rangers v Celtic Scottish Cup final.
He was also fined £3,500 by the Faculty of Advocates.
In 1995 Paul Gascoigne, playing for Rangers, landed himself in trouble in a pre-season Old Firm friendly as he mimicked playing the flute, infuriating many Celtic supporters who saw the act as a loyalist symbol.
Gascoigne, who was unaware of the significance, was disciplined by the Scottish FA.
In 2002 Celtic made an unprecedented appeal to a "vocal minority" of its supporters to stop chanting IRA slogans during games.
Many Old Firm matches have been marred by crowd trouble over the years, and there has also been violence away from the football grounds.
In 1995 Mark Scott, who was wearing a Celtic shirt, was stabbed to death as he walked past a pub full of Rangers fans.
Five years later several of his friends helped set up an organisation, Nil By Mouth, which campaigns against sectarianism.
Both clubs have made considerable efforts to end sectarianism among their fans.
Last month Rangers chairman David Murray said the consequences for the club of fans continuing to sing sectarian songs would be grave.
That followed a Uefa fine for the Ibrox club for "discriminatory chanting".
Last year Celtic and Rangers jointly launched a project to tackle bigotry and sectarianism in the west of Scotland.
The two clubs have ploughed tens of thousands of pounds into the Old Firm Alliance initiative, which aims to educate youngsters and the wider community about the ills of sectarianism through a series of football courses.
Later this year Scotland's First Minister Jack McConnell - who describes the issue as "Scotland's secret shame" - will host a second summit on sectarianism in an effort to eradicate bigotry.
A similar event was held in Glasgow in 2005, bringing together about 30 organisations.
Celtic and Rangers football clubs, the Catholic Church, Church of Scotland and the Orange Order are all involved in the project.
It is against this background that Artur Boruc's actions have to be judged.