If you get the “ran out of buffer space error” it is likely that the page you are testing has a huge viewstate. This will lead to a buffer space error in Hpricot which Raakt uses to build the document tree. In newer versions of Raakt, this is set to 256 Kb. If this isn't enough you need to specify something larger for the Hpricot.buffer_size variable in raakt.rb.
The Raakt source is available from a Subversion repository at Rubyforge. If you have installed Subversion you can get the latest revision by typing this in a command window:
svn checkout svn://rubyforge.org/var/svn/raakt
Maybe I missed it. If you know of an accessibility test that all documents should pass I would be grateful if you add it as a feature request.