Belfast City Council failed to fly the Union flag on a designated day for Prince William's birthday - resulting in a stern telling off from Ruth Patterson.
The flag is only flown from Belfast City Hall on 18 designated days per year since the council controversially decided in December 2012 to restrict the number of days.
One of these days is June 21, to mark Prince William's birthday.
But DUP councillor Ms Patterson complained at this week's council meeting that she had turned up for a loyalist flag protest that day to find the flag was not flying.
"Saturday week ago I came down for the weekly flag protest at City Hall, knowing that it was the Duke of Cambridge's birthday therefore the Union flag should have been flying above the building," she told the council chamber.
"Now whenever I arrived down I was confronted with a sea of 'why isn't the flag up?'"
She said she rang the "emergency number" at Belfast City Council, and a member of staff raised the flag.
"It was very quickly addressed and for that I am very grateful. I know that the people out there on the streets were also very grateful," she said.
"But it is something that should never have happened in the first place.
"It was the Duke of Cambridge's birthday. It is a designated day. Therefore the flag should have been flying."
Following Belfast City Council's flags decision in 2012, loyalists took to the streets to protest, some demonstrations involving violence during which many police officers were injured.
Hundreds of people were convicted through 2013 and the flags issue remains one of a number of outstanding peace process disputes that politicians have failed to reach agreement on.
Ms Patterson added: "Now I just bring this for notation, but I certainly wouldn't like to think that this would ever happen again.
"We fly the flag too little on Belfast City Hall now so therefore when there is a designated day the flag should be flying high."
A council spokeswoman said it was a mistake: "There are two lists for flag days issued by the Department of Culture Media and Sport and the DOE, and we mistakenly referred to the wrong list.
"Once Alderman Patterson pointed out the error by contacting our 24 hour control room the Union flag was raised."
Today Northern Ireland's political leaders are meeting at Stormont for renewed talks, six months after marathon negotiations chaired by former US diplomat Richard Haass ended without agreement.
A recent survey by Northern Ireland's two universities said more Protestants prefer the Union flag to be flown on public buildings only on designated days rather than all-year round.
The Life and Times Survey, carried out by also found that community relations have plummeted in the wake of loyalist flag protests and related violence.
The dates for flying the Union Flag on UK government buildings in 2014 are:
9 January Birthday of the Duchess of Cambridge
20 January Birthday of the Countess of Wessex
6 February Her Majesty’s Accession
19 February Birthday of the Duke of York
1 March St David’s Day (in Wales only)
10 March Birthday of the Earl of Wessex
10 March Commonwealth Day (second Monday in March)
17 March St. Patrick’s Day (in Northern Ireland only)
21 April Birthday of Her Majesty the Queen
23 April St George’s Day (in England only)
9 May Europe Day
2 June Coronation Day
10 June Birthday of the Duke of Edinburgh
14 June Official celebration of Her Majesty’s birthday
21 June Birthday of the Duke of Cambridge
17 July Birthday of the Duchess of Cornwall
15 August Birthday of the Princess Royal
9 November Remembrance Day (second Sunday in November)
14 November Birthday of the Prince of Wales
20 November Her Majesty’s Wedding Day
30 November St Andrew’s Day (in Scotland only)
Volatile issue a perennial source of division through history
By Claire Williamson
Flags remain a contentious and volatile issue in Northern Ireland.
While the law has attempted to regulate the controversial issue, it has been the source of many protests.
Northern Ireland has not had its own flag since 1972.
The flag of the Government of Northern Ireland was officially named The Ulster Banner and was used from 1953 to 1972.
Based on the flag of England and the flag of the province of Ulster, it had the Red Hand of Ulster in a white star – points of the star represented the six counties of Northern Ireland.
But when the Northern Ireland Government was dissolved under the Northern Ireland Constitution Act 1973, the flag ceased to carry any official status. Also known as the 'Red Hand of Ulster' or the 'Ulster Flag', it is still used by some unionists and sporting organisations
The only official flag to be flown is the Union flag, which is an amalgamation of the crosses of St George, St Andrew and St Patrick representing England, Scotland and Ireland.
In 1954 the Northern Ireland Parliament passed the Flags and Emblems (Display) Act. This Act gave police the power to remove or order to remove any emblem that could lead to a breach of the peace – but it was repealed in 1987.
The issue was recognised in the 1998 Good Friday Agreement, which noted the "sensitivity of the use of symbols and emblems for public purposes, and the need to ensure that such symbols and emblems are used in a manner which promotes mutual respect rather than division".
The most recent legislation on the contentious subject was the Flags (Northern Ireland) Order 2000, which gave the Secretary of State for Northern Ireland power over the regulation of flags at Government buildings and on designated days.