derailment

(countable, uncountable) The action of a locomotive or train leaving the rails along which it runs.
(countable, uncountable) An instance of diverting a conversation or debate from its original topic.
(countable, uncountable) An instance of thwarting or frustrating something.
(countable, uncountable) A pattern of discourse (in speech or writing) that is a sequence of unrelated or only remotely related ideas.