A design competition in early 2000 in the UK which challenged university students to create a "convincing lie" was effectively sabotaged when one of the entrants convinced the universities involved that the competition had been cancelled. The student had created suitably convincing headed notepaper from the poster used to advertise the competition and had mailed phony cancellation notices to each of the participating universities.
I can find no record of the outcome of this story, but if this person was not awarded the first prize, there is no justice.
While I’m aware that this story contains many of the hallmarks of an urban legend, I have been able to trace the Brighton University Professor, George Hardie, who is quoted in the source article. You can see a picture of the Prof. at http://www.bton.ac.uk/audience/former/pdf/10/page16.pdf
The original story appeared in the London Evening Standard, March 29 2000, and is quoted in FT 139.