There is a farm cited as an ancestor origin by multiple deceased genealogist. Its a common POI that people with this shared ancestor try to find. Finding it could solidify the established line theory and finally debunk a small ancestor fraction's alternate theory.
Different LLM searches keep returning the same colony with invalid/made up references that don't even cite a partial name match. Manual searches throughout that area returns nothing as well. But since the LLM said so, a century+ of various research and work by professional genealogist gets severed from a crowd sourced public tree because "AI" returned a colony name that ended up helping the alt line.
I did eventually find what I believe is the origin farm, and it strengthens the history written by previous genealogist -- I tried to send the information to the tree maintainers and was outright ignored. LLMs fabricating locations apparently beats a listing in the National Heritage List for England of the exact place name, buildings from that time, and in one of the areas the larger english family is known to have been present.