I don't think there's a word for the opposite phenomenon, that happy moment when you come across some fact that actually strengthens your argument, made on other grounds: something you'd gladly have included in your original piece had you only known of it at the time.
I had an example of that this week. I've been doing some research on the Clarendon Chaucer, Tolkien's never-finished collaboration with Geo. Gordon, as part of my Kalamazoo piece, and ran across a passage that'd escaped my notice before. It seems that in May of 1951, when Tolkien turned over all materials for the long-abandoned project to Oxford University Press in hopes they might be able to find another Middle English scholar to complete the book, Tolkien "has suggested that Dorothy Everett might complete the book" but the head of the Press "does not want to distract her from another project" (Scull-Hammond CHRONOLOGY, p. 375). Given that earlier it had been suggested that E. V. Gordon, Tolkien's collaborator on the SIR GAWAIN & THE GREEN KNIGHT and several other unfinished projects, and fellow future Inkling J. A.W. Bennett might take over the project, that Tolkien wd suggest Everett for the job seems to indicate that he had a high regard for her ability to do a good job as his collaborator. I had argued in my piece "The Missing Women: JRRT's Lifelong Support for Women's Higher Education"* that Tolkien was a strong supporter of women taking advanced degrees and pursing medieval scholarship (as opposed to the much more dismissive attitude of his friend C. S. Lewis), and this provides just another example which I'd have been glad to include, had I noticed it in time. As for Everett herself, I need to look up more about her. As it is, she flits through the great Scull-Hammond CHRONOLOGY without making much of an impact, leaving behind the general impression that she was someone who went to a lot of the same committee meetings as JRRT. I do know she died just two years after being mooted for the Clarendon Chaucer project, so that may have had something to do with its ultimately being abandoned.
*recently published in PERILOUS AND FAIR: WOMEN IN THE WORKS OF J. R. R. TOLKIEN, ed. Croft & Donovan