The problem was that, from example from a test: WikiPage.new() would make a new instance with content => nil. When invoking WikiPageController -> show show.html.erb would call "@page.content( nil)", which would call "wiki_show_attachments( nil )", which would call "nil.gsub", which would bomb. I think the optimal solution would be to set the default for the content column to an empty string inside the migration, however I did not succeed in implementing that (MySQL doesn't seem to allow default values for "TEXT" type columns?). And instead of peppering all methods with "foo( arg || '')" I though setting it in before_save would be the second best solution.
Currently when the user clicks on a link, she gets to the no.html.erb page which only links to a "wiki/new" action without the newly to be created page title.