some_spec.rb

stub_const('Something', object_double(Something))
unrenderable_exception = instance_double(Exception)
allow(unrenderable_exception).to receive(:my_custom_method_ive_added_to_exception).and_raise
allow(Something).to receive(:some_method).and_raise(unrenderable_exception)

# code that triggers the call to Something.some_method
# assertions that check for the result

# Essentially I have some custom exception handling logic in my_custom_method_ive_added_to_exception
# and want to test that if it goes wrong my 'fallback' exception handling kicks in.
# Unfortunately, the above doesn't seem to work. Instead of using my 'unrenderable_exception' mock object,
# I see a RuntimeError as the raised exception.