On April Fool's day, I called my friend to tell her that there was a news story about how the city water supply was contaminated.  Since the water in the pipes currently wasn't, we were supposed to fill pitchers and empty bottles with water.  I was on my way to her house to help her since she had two small children. 

By the time I got to her house, she had filled her bathtub and all her pots and pans with water!!!!  She had convinced herself that there was really a tornado coming and that I was trying to get her to prepare without freaking her out.  (although why she'd need water for a tornado was never explained - she just said that was her first thought!)

She was so relieved that nothing was wrong that I didn't get in trouble!  That might have been the best prank yet...