In 1986, volunteers in Cleveland inflated 1.5 million helium balloons to break the Guinness World Record and raise funds for the United Way. What could possibly go wrong? Despite threatening weather, the balloons were release from the huge netting, and many lovely pictures were taken.
Then the "asteroid field" of airborne debris clouded the sky, shut down a runway at a local airport, interrupted Coast Guard attempts to rescue a pair of fisherman, spooked some prize-winning horses, and generally made a mess of un-biodegradable [edit: apparently the balloons were biodegradable, fwiw!] garbage on land.