A company in the States is selling corrupted files for students to hand in as homework assignments. Corrupted-files.com will sell you, for $3.95, an Excel, Word or Powerpoint file that won't be able to be opened.
The idea being, the company says, that you use the time before the professor tries to open it to write the real essay. When you get the email saying "I can't seem to open your file", say "Oh, I'll try and send it again" and send the finished essay.
For $10, the company can corrupt more formats, including mp3, swf, and zip. The full list is on their site. If you're in a bind without having had time to do your work, then maybe it's worth a shot. But we wouldn't recommend trying it more than once.