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Well, this is sort of an answer, I just have no memory of the participants or the area of mathematics, just that it may have been in Yemon's world. But 25 years ago. Postdoc $\varepsilon$ met established Professor $Y,$ and, pleased to meet the great man, proceeded to give a sketch of the proof of his main result. (I did not use $X$ because...) $\; \; Y \;$ listened for a while, then said, "That's a very nice result, let me see if I can disprove it." $Y$ then proceeded to rattle off a sequence of examples and figured out in what way each was not a counterexample to the new result. $\varepsilon$ said later that he learned more about his result in that 20 minutes than in all of graduate school. He went on to work with $Y$ for some time.

This has seemed the right response to various MO things, I have just been embarrassed about remembering no names, places, or much else. It still works as urban legend, though.

EDIT: it is not clear to me how I came to hear of this, but Henry Cohn has pointed out that this must be Keith Kendig and Hassler Whitney, see the second page in this pdf: HASSLER_WHITNEY a short essay by by S. Swaminathan, also in French on page 3 of the pdf.

Well, this is sort of an answer, I just have no memory of the participants or the area of mathematics, just that it may have been in Yemon's world. But 25 years ago. Postdoc $\varepsilon$ met established Professor $Y,$ and, pleased to meet the great man, proceeded to give a sketch of the proof of his main result. (I did not use $X$ because...) $\; \; Y \;$ listened for a while, then said, "That's a very nice result, let me see if I can disprove it." $Y$ then proceeded to rattle off a sequence of examples and figured out in what way each was not a counterexample to the new result. $\varepsilon$ said later that he learned more about his result in that 20 minutes than in all of graduate school. He went on to work with $Y$ for some time.