So far, I've pulled two shenanigans:
  1. I sent email to lots of friends saying "It's that time of year... hope you remembered to set your clocks forward!". Nobody fell for it.
  2. I created Harry Potter and the Order of the Phoenix on our local library server, then sent a message to local librarians saying how wonderful it is, it's #1 on the bestseller lists, etc. Nobody bit -- I didn't hear a single peep from anyone about the book, and I didn't get any reactions to the e-mail in which I confessed it was a hoax. Maybe the stereotypes about librarians are true...

For posterity's sake, here's how the library record appeared:

     AUTHOR    Rowling, J. K.

      TITLE    Harry Potter and the order of the phoenix by J. K. Rowling ;
               illustrations by Mary GrandPre. 

    EDITION    1st of April ed.

 PUB. INFO.    New York : Arthur A. Levine Books, 2002 

  DESCRIPT.    410 p. : ill. ; 24 cm.

    SUMMARY    Harry Potter, a fifth-year student at the Hogwarts School of
               Witchcraft and Wizardry, is determined to defend his alma mater
               from the evil Lord Voldemort by joining a mysterious magical
               fraternity, despite the cost to friends and family. 

      NOTES    Sequel to: Harry Potter and the goblet of fire.

   SUBJECTS    Wizards -- Juvenile fiction
               Magic -- Juvenile fiction
               Schools -- Juvenile fiction
               Secret societies -- Juvenile fiction
               Wizards -- Fiction
               Magic -- Fiction
               Schools -- Fiction
               Secret societies -- Fiction
               England -- Fiction
               England -- Juvenile fiction

      GENRE     1) Fantasy fiction.

  ADD. NAME     1) GrandPre, Mary ill.