So far, I've pulled two shenanigan
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.
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.