For a nice example of how articles can gradually deteriorate when the vandals overwhelm the vandal-fighters, see:

http://en.wikipedia.org/w/index.php?title=Network_topology&curid=41413&a...

I particularly love how, after a vandal had replaced a section with the sentence "The topology system was originally invented by Robbie Cowan", someone comes along and tags the statement with "citation needed"...