Flash message not being displayed

I can't seem to get a flash message to display in my app after catching
an exception.

I have a method that loads the current user based on the current
subdomain, the current_subdomain_user method throws an
ActiveRecord::RecordNotFound if it can't find a user for a particular
subdomain.

I have a before_filter that calls this method and catches any record not
found exceptions, sets a flash message and redirects.

The problem is that the flash message isn't being displayed, I can't
understand why not. Any help would be much appreciated. Below is the
before filter

  def user_required
    begin
      current_subdomain_user # this method will raise an exception if
the current subdomain user can't be found
    rescue ActiveRecord::RecordNotFound
      flash[:error] = "Couldn't find the user #{current_subdomain}"
      redirect_to root_url(:subdomain => false)
    end
  end

Do you have something like

<%= flash[:error] %>

in your layout or view file?

Not sure if you got this answered but a guess would be because the
flash hash is persisted only across the same domain... your subdomain
doesn't see the flash message.