Leo over at Neural Gourmet just emailed a few links to this group of people called Prangstgrup who apparently go around to colleges, pose as students, and perform pranks in the libraries and lectures.  I love practical jokes, and what I love about these practical jokes in particular, is that they're really at no ones expense.  Check them out.

• A prank in a lecture.
  
• A prank in a library.