Element not found in the cache - perhaps the page has changed since it was looked up (Selenium::WebDriver::Error::StaleElementReferenceError)

I am trying to click all links on stackoveflow horizontal menu (Questions, Tags, Users, Badges, Unanswered). I have this code but this clicks on first link (this link is Questions), then prints 1, and after that raises error. What could be problem with this?

require 'watir-webdriver'

class Stackoverflow
  def click_all_nav_links
    b = Watir::Browser.new
    b.goto "http://stackoverflow.com"

    counter = 0
    b.div(:id => 'hmenus').div(:class => 'nav mainnavs').ul.lis.each do |li|
      li.a.click
      puts counter += 1
    end
  end
end

stackoverflow = Stackoverflow.new
stackoverflow.click_all_nav_links

Error message is: https://gist.github.com/3242300