The death of Osama bin Laden not only sent waves across the Internet, but false information as well.
"I mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy." - Martin Luther King, Jr
A simple Google search reveals that the phrase cannot be traced back to King. Placing the phrase in quotation marks and searching it yields no confirming sources.
The Atlantic suggests the phrase was started by a 24-year-old Jessica Dovey, who is an English teacher in Japan. According to them, Dovey added the phrase to state her own opinion while adding a subsequent MLK Jr. quote, but accidentally linked the two.
Another quote, this time attributed to Mark Twain, also spread quickly. However, as Gizmodo points out, this too was a fake.
"I have never killed a man, but I have read many obituaries with great pleasure."
UPDATE: Apparently the misappropriation was on the part of someone who copied Dovey's post, not her herself, according to the updated Atlantic article.