The stacks creeper is the bibliothecal equivalent to the the 'peeping tom,' except that instead of peeping through the curtains of a bedroom, the offender spies on someone from behind a bookcase, or book stacks. Patrons who suspect someone of 'stack creeping' are strongly encouraged to report that individual to a security guard or the front circulation desk. Like eating, drinking, smoking, and looking at pornography on the computers, stacks creeping is not allowed at Shields Library.
There is also the more sinister, and far more annoying, Stacks Stalkers. These are frequently girls who use the books and shelves as an easy way to peek and spy on a guy they think is cute. Their loud giggles can be heard up to 20 feet away, and they frequently walk off red-faced when spotted. 
...creep the stacks. November 2007 Election/Measure P
