“Don't believe everything you read on the Internet.”
This quote is frequently purposefully misattributed to Lincoln or others long dead before the age of the internet in order to emphasize its point using humour; not all such attributions, or other claims, found on the Internet are as obviously flawed. " "Cite and sound: the pleasures and pitfalls of quoting people", by Tom Calverley, The Guardian (14 October 2014) http://www.theguardian.com/media/mind-your-language/2014/oct/14/mind-your-language-quotations
Variations:
Don't believe everything you read online.
Don't trust everything you see on the Internet.
Everything you read on the Internet is true.
The trouble with quotes on the internet is that you never know whether or not they're genuine.
Misattributed
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Abraham Lincoln 618
16th President of the United States 1809–1865Related quotes

“Believe what you like, but don't believeyou read without questioning it.”
Source: Questionable Creatures: A Bestiary

“Reading is important - read between the lines. Don't swallow everything.”

You Can Lead an Atheist to Evidence, But You Can't Make Him Think (2009)

answering whether he'd learned from the Internet that a men's room in Minn. airport is a hot spot for anonymous gay sex, in interview with Matt Louer; October 16, 2007; http://www.msnbc.msn.com/id/21361806/ http://www.youtube.com/watch?v=fwx8sV1LV1A
FACT CHECK: Email constitutes use of the Internet; he has served on the Congressional Internet Caucus; he has advised in a recent op-ed to 'do a Google search on "mission creep"'; he has been a co-sponsor of a national Internet safety bill; he has received a 2007 Internet Keep Safe Award http://www.msnbc.msn.com/id/21361806/

“Don't believe anything you read on the net. Except this. Well, including this, I suppose.”