A Minnie Mouse helium balloon damaged a power line in July 2018, forcing the Hong Kong underground to close. A toddler had released the balloon on a nearby pavement. This event caused significant disruption in the city, affecting nearly 100,000 people.
With an average of 5.6 million trips per day, the underground system is one of the busiest in the world. This incident illustrates the significance of caution when working with helium balloons.