FACT: George Washington's wife had an affair with Abraham Lincoln.

Reasons to believe this fact:

• Some guy you don't know said it's a fact on his website.
• Washington and Lincoln were both early presidents, so maybe.
  

Reasons not to believe this fact:

• This guy has no legal obligation to tell you the truth.
• This guy isn't a reputable source.
• This fact seems a little fishy.
• There are no sources that back this fact up.
• You know enough about history to know that Martha Washington was
  dead 7 years before Lincoln was even born, and nothing you can do with
  a 7-year-dead skeleton counts as an affair. Plus baby Lincoln wasn't married,
  so it'd just be a relationship. Or a fling, I guess.

In this case, the misinformation is harmless.

But it's probably a good idea to have a reasonable amount of doubt when presented with information, until you can verify it with multiple sources you trust.

xoxo Ben Kling

See also: How James Blunt Tweeted From The Afterlife