thought experiments, incoherency

When engaged in the philosophical dialectic, we should be careful to avoid proving our point on the basis thought experiments regarding scenarios that are far-fetched.  I think everyone would grant that no thought experiment that involves two contradictory claims can be allowed.  And pretty much everyone would agree that a thought experiment involving an event that is highly improbable but quite possible should be allowed.  Besides these two alternatives there is a thought experiment of a third type: it asks us to imagine something that may in fact be impossible in the same way that it's impossible that "green ideas" would "sleep furiously": not a contradiction between two statements but an incoherency in one statement.  When we are unsure of what the words mean; when determining what those words mean; and when one of the candidate meanings would mean, if accepted, that the scenario in our thought experiment is incoherent, then making use of such a thought experiment to shore up belief in a controversial thesis is an example of question-begging.