Overthinking the problem, or another post where I forget the demographics of my readership.

class A
  attr_accessor :check_errors

  def initialize
    @check_errors = Proc.new do |something|
      return something if something.kind_of? Exception
      if something.respond_to? :each
        bad = something.detect { |thing| thing.kind_of? Exception }
        return bad if bad
      end
    end
  end
end

class B < A
  def maybe_erronous
    # do stuff...

    self.check_errors(questionable)

    # do more stuff...
  end
end