It was supposed to mark the end of an era after crowds gathered to watch the start of demolition work to five Leeds tower blocks.
But the operation had to be halted within minutes after a tumbling boulder bounced over a security fence and hit a pensioner in the chest.
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Marian Boyle had stopped by to watch the demolition as she used to live in one of the blocks herself.
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But the 64-year-old bent over double after the chunk of concrete hit her in the freak accident.
Neighbours rushed out to bring her a chair while other bystanders called an ambulance.
Paramedics arrived and treated Ms Boyle for shock.
The volunteer at the nearby Holbeck Foods told the YEP: "It's a good job it hit my chest rather than my head. It is sore and will probably just be bruised. I don't think anything's broken but I have got heart problems so I just want to get checked."
Friend Sarah Gamage, 25, was taking photos at the time.
She said: "I brought her down because Marian used to live in the flats. Everyone has been so nice and helped calm her down. I was worried she would have an asthma attack and she also suffers with her heart."
A giant nibbler-style machine was geared up to begin taking the top floor off Holbeck Towers while surrounded by security fencing.
But after five minutes work was halted and the Health and Safety Executive will now launch an investigation.
A spokesman for the council said they were using this method of demolition instead of blowing up the buildings as this was too costly.
He said: "If you blow them up we would have had to evacuate all the nearby houses and close the motorway. I don't know how long it will be until the demolition work can start again - it depends when the HSE can come and look at the site."
Coun Matthew Lobley, deputy executive board member for housing, said that when similar demolition was carried out in Little London there were no problems.
He said: "All the health and safety work will have been carried out absolutely as it should have been so this unfortunately is a freak accident."