Widespread quotation misinformation predates the internet. William Safire got himself embroiled in just such a scandal. He had used a quotation attributed to Edmund Burke in a column, and then a reader wrote him a letter asking when and where Burke had said said quotation. Safire checked his copy of Bartlett's Familiar Quotations, 14th edition, which placed the quotation in a certain letter. Safire sent a reply. The guy wrote back: he'd examined that letter, and the quotation wasn't in it. So then Safire embarked on a fruitless quest to prove that Burke really wrote, " The only thing necessary for the triumph of evil is that good men do nothing."*
The 16th edition of Bartlett's lists this under Burke as "Attributed."
Now it is possible that Leo from Vinci said something that could be translated as the above quotation and that somebody found it somewhere written down by Leo on some forgotten manuscript in a hidden archive. So if you are that somebody, I wish you would identify yourself and cough up a citation.
* William Safire On Language, NY: Times Books, 1980, pp 224-227.
UPDATE 11/08/11 I find a reference to this problem in Wikiquote.
ANOTHER UPDATE 4/3/15 Quote Investigator did a thorough investigation!