I'll admit it. When I see a good quotation, I often use it (i.e., retweet it) before I check out its source and authenticity. A good example would be the recent Martin Luther King quotation that spread after Osama bin Laden was killed.
The story behind this misattribution, according to The Atlantic, is that it went viral after Jessica Dovey posted a status update on Facebook but added MLK's quotation at the end and the two became associated:
From Jessica Dovey:
"I will mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy."
From Martin Luther King Jr.:
“Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.”