When I first began into geneaology, I was astounded and delighted with the amount of information I found in already-created family trees online. With the click of a few buttons, someone else's entire gedcom could be integrated with me. Hundreds - or even thousands of years of ancestry scored in a matter of a few minutes!
Ancestry.com, ftdna.com and other similar genealogy research sites have made it easy to research, publish, share and copy family trees, even if you have zero genealogy research experience. They have also made it terribly easy to amplify the errors of the inexperienced exponentially.
So, then came the part where I realized most of what I got from others was trash. I spent days - nay, weeks, weeding out individuals and relationships that were not possible or were not borne out by proof. Amidst all my grumbling and complaining and kicking myself for ever having used them to begin with, I started to see some common threads between the inaccuracies. I think much of it comes from a fundamental misunderstanding of what constitutes proof versus evidence, some of it is wishful thinking and some of it is just an honest mistake.
Whatever combination of these it is, some "genealogists" leap over impossible hurdles to make connections that are not there. Mothers give birth to babies posthumously or at the age of two. Father's father children years after their own death. Mothers have children a mere two months apart. Mothers and fathers live to a hundred to give birth to a baby. There was a king or queen or famous person once, who lived in the same city and they had an unknown child when they were 205 years old who was my 32nd great grandfather. What?
I have, in my own family, several examples of these myths becoming fact without any real logical foundation. One of them is the myth of Jordanus de Sheppey who is, 90% of the time, mistakenly reported to be the son of King Harold II.
The Norwood line can be traced fairly unerringly back to a man named Jordanus de Sheppey. The myth in the Norwood clan (in short) is that during his life, Harold Godwinson (King Harold II) had a son who hid away on the Isle of Sheppey and changed his name to Jordanus, becoming Jordanus de Sheppey. There is absolutely nothing in the way of proof tying King Harold to the Norwood family and it is actually mathematically impossible that Harold's son is 'our' Jordanus.  But despite the story failing basic proof and logic tests, it continues to be propagated - and even argued - mostly because, I think, being related to a king is better than being related to a.... brick wall.