Something static in RuntimeError

If you have two Net::SFTP::Exceptions in one process, the error messages get concatenated. This is probably because the message method of RuntimError (or of something above) returns something static which gets reset only in the initialize method:

class MyError < RuntimeError
  def initialize(response)
    @response = response
  end

  def message
    m = super
    m << @response
  end
end

2.times do
  begin
    raise MyError.new('error')
  rescue RuntimeError => e
    puts e.message
  end
end

Which outputs:
MyErrorerror
MyErrorerrorerror

If you call super from the initialize method or change the last line in the message method to “m + @response” the outputs are more as expected:
MyErrorerror
MyErrorerror

errorerror
errorerror