'There's dihydrogen monoxide in the water'
In 2002, two local radio DJs in Kansas City decided to try a science-inspired approach to April Fool's Day pranks, telling their listeners that the tap water in their local area had been found to have high levels of a “naturally occurring substance known as dihydrogen monoxide". Side effects of the chemical, the pair said, included urination and wrinkled, "prune-y" skin.
But after concerned members of the public began ringing the water department, dialing 911 and speculating about whether or not the "contamination" could be a terrorist attack, the radio station was forced to issue a grovelling apology.
Dihydrogen monoxide, in case you're wondering, is the scientific name for H2O - also known as plain old water.