Some of these false sources, have a large amount of correct well, known information that masks when they go off into fantasy.The Forgotten Monarchy of Scotland is one interesting example - I suppose the title itself should be a good clue.
I really like John's idea. It does not have to be all inclusive list, but could start with a list of most common bad sources cited here at WikiTree (with sources explaining why/how they have bebunked. that page could then be linked in the Research notes section of a profile explaining when not to to do to the profile in question..
For example, as I understand it much of the pre-MacAlpin Scottish history is probably bunk, but it is sprinkled with people that are known to exist from records, connecting the to a single linear set of ancestors is where it apparently becomes fictional. While not an expert here, I believe there is academic research behind these well known early Scottish royal pedigrees being an product of competitive Scottish nationalism at a time when English sources were citing a deep pedigree of the English Kings.