1.5 million balloons

Viral Forest:

In 1986, organizers with United Way of Cleveland thought they had the perfect idea to generate a little publicity and create a beautiful spectacle in the process. With a crowd of volunteers working all hours, they filled 1.5 million helium balloons, and released them all at once. Unfortunately, they had no idea the terrible consequences they would unleash by doing so, and their tragic mistake led to the deaths of two people and millions of dollars in damages through lawsuits.

Check the article for some great photos.