We're told it was started deliberately.
Rangers Play Celtic At Hampden
Celtic and Rangers meet for the first time in over a year today, with police warning fans to be on their best behaviour.
Any pub screening the Scottish Cup semi-final is deemed to be a "football ground'' in law, Police Scotland said, and anyone who commits an offence could be given a football banning order and barred from attending future games.
Chief Inspector Stephen Stiff said a ''small minority who spoil the occasion for others'' will be targeted on Sunday when Rangers and Celtic meet at Hampden.
Officers have also visited known domestic abusers ahead of the game as they said history shows a rise in drink-fuelled violence around an Old Firm tie.
Police have been to pubs to ''offer safety advice'' and visited ''known violent offenders'' who pose a risk of disorder.
Patrols will be stepped up on Sunday, focusing on pubs and the conduct of travelling fans and preventing disorder.
Mr Stiff said: ''This will be the first meeting of the two teams for some time and has the added pressure of being a cup semi-final. History indicates that there is a rise in alcohol-related violence, including domestic violence, around an Old Firm game and as such we have been proactive in highlighting this to licensees across the region who will be screening the match live this weekend.
''Police Scotland officers have already made visits to licensed premises to remind staff of their licensing conditions and to offer safety advice. Officers on Sunday are fully aware of those premises where the game is being screened and patrols will be directed accordingly.
''We also need local supporters to understand the wider consequences of their actions if becoming involved in football-related disorder.
''Offenders don't necessarily need to attend the game to attract a football banning order if convicted for a football-related offence, and we will assess circumstances closely and pursue all legal avenues to prevent future offending.''
Celtic won 2-0 in the last match between the sides in the Scottish League Cup last February.
With Rangers winning the Scottish Championship, Old Firm games will become a regular fixture in the Scottish Premiership again from next season.
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