I tend to believe that trying to correct errors on bad sites like Wikitree and Geni does more harm than good. Such anybody-uploads-anything-they-want sites are never going to be worthwhile without reasonable quality control...
@HWinnSadler, I detached Avelina Hesding's wikitree profile from King Alexander of Scotland, thanks.
@Stewart Baldwin, I hope that your low opinion of wikitree is based on out-of-date information. Wikitree has, for the past couple years, been blocking people from editing pre-1500 profiles unless they have obtained a "pre-1500 badge," which requires them to demonstrate basic competence in using sources. The accuracy of the wikitree profiles of my own medieval ancestors is steadily improving.
Recently I did a lot of work on the wikitree profiles of the ancestors of my presumed ancestor Jane (Machell) Rich. I invite you to criticize their accuracy -- a five-generation chart is at https://www.wikitree.com/genealogy/Machell-Family-Tree-6
If anybody edits any of these profiles (going back five generations or so), I will receive an automatic notification, so I can keep an eye on them. However, this lineage connects to the earlier Bellingham/Tunstall families -- profiles that I did not create and haven't worked on, so I can't vouch for their accuracy. The same goes for the even earlier Radcliffe connection to the Machells' Kirkby/Urswick line.
A different example, that I did a lot of work on three years ago, is the ancestry of my ancestor Humphrey Prideaux (his mother was a Poyntz, going back to Magna Carta baron William Malet). A five-generation chart is here: https://www.wikitree.com/wiki/Shapton-3 People just don't come along and try to add fantasy parents; these profiles have been quite stable. I'm not saying that they are error-free. I AM saying that I am keeping an eye on what wikitree does with this particular ancestral line, and if you or somebody else comes along to point out errors, I can jump in and fix things.
A third example is Philippa Bonville, wife of William Grenville. British genealogists seem to have been consistent in concluding that Philippa was the DAUGHTER of William, Lord Bonville; while American genealogists Weis and Richardson concluded that she was William's sister. After long discussion, William was "reinstated" as Philippa's father at wikitree, using wikitree's "uncertain" feature to alert people to the lack of documentary proof -- unless you're willing to accept the heraldic implication in that stained glass window with Bonville impaling Grenville, which is a new argument that needs to be thought through and digested. Maybe, in the long run, there will be a consensus among medieval genealogists on that one.
One last example: Years ago Ronny Bodine posted a reworked genealogy of the Champernoun family here at SGM. I used that to thoroughly overhaul the Champernoun lineage at wikitree, adding Bodine's research to wikitree's earlier Champernoun profiles and making Bodine's work more accessible in the process.
At this point, wikitree may or may not be a reliable source for a given medieval family -- as always it depends on the sources cited in any individual profile. On the other hand, I have found that wikitree is an excellent tool for presenting and preserving and improving on my own research.