What is the most eloquent way to transition into pointing out a major flaw in someone's thinking?

Question

There is phrase that I really liked and was quite articulate, but I can't think of it. This is in response to a debate when analyzing one's position or paradigm. Other ways:

• crucial fallacy
• serious mistake
• flagrant error
• pivotal blunder
• grave misunderstanding
• critical misconception

Answer 1

Perhaps:

• dire mistake
• fatal flaw

Or if not, I guess you could find the adjective and noun you're looking for in a good thesaurus.

Answer 2

What is the most eloquent way to point out a flaw in someone's thinking?

Since thinking is such a broad activity it is difficult to believe that there is one single word which hits this target infallibly.

Here is a list of fallacies from the "Internet Encyclopedia of Philosophy". There is also this one from the "Nizkor project" which has been knocking around the internet for some time.

Answer 3

My boyfriend reads a lot but only attended one semester of college so he knows words that he hasn't heard pronounced. When he mispronounces one, I wait until we are alone and say something like, "I have only heard that word pronounced X.". That way he can can save face, and I am not over-assuming, because some words are pronounced in more than one way. To be sure, there have been times when my pronunciation wasn't the most common either.

I think the same tact should be taken with any difference of thought, because you can not always be certain you yourself have all the information. "Have you considered" is another respectful way to present a difference of opinion.