The art of quotation is fraught with difficulties. If the source of a quotation is not given it can lead readers to think that the author using the quotation originated the thought or that he is being dishonest. Some people are thought to have said certain things, but there is no evidence of these words in any of their surviving writings: when this is the case, the words have merely to be attributed to them.

Many quotations are routinely incorrect or attributed to the wrong authors, and quotations from obscure writers are often attributed to far more famous writers by lax quoters. Good examples of this are Winston Churchill, to whom many political quotations of uncertain origin are attributed, and Oscar Wilde, who has said far more witty things than he possibly could.

Deliberate misquotation is very common either because the misquotation is better known than the original or simply because the misquotation fits the situation better. Possibly worse than misquotation is deliberate misinterpretation, where an author's words are taken out of context and are used to support a position or idea that the author would never have agreed with and was not the author's intention. This can be especially problematic with playwrights and authors of fiction who do not necessarily agree with the sentiments of their characters.

Source:  Wikipedia