The thin line between fact and fiction.
When James Frey published his memoir, A Million Little Pieces, he was all the rage. Oprah chose this for her book club, and it sold nearly 2 million copies in 2005. Then, Frey was "outed." His book became known as A Million Little Lies. What happened?
Frey made the critical mistake of writing things that weren't true AND could be documented as lies. Don't all memoirs contain some "untruths"? Probably. A memoir is a snapshot of someone's life. A period of time that they document from memory. Since we all have imperfect memories, it stands to reason that any book written purely from memory would contain some erroneous information.
That's expected and appropriate, if it's not intentional and documentable. If you say you've been in the military, but the military has no record of this and no one recalls your having been a soldier, then you're in trouble. If you say that your family drove across the United States in a blue car, but it was really tan, well, that's a reasonable mistake. It would be pretty hard to "forget" that you weren't in the military and fairly easy to document this On the other hand, it's much easier to forget the color of a car that you took a trip in many years ago, and that would be harder to document. Readers would expect and forgive an inaccuracy such as the latter one.
However, as Frey and many others have found out, they don't forgive it when authors lie. The Smoking Gun investigated and found many documentable inaccuracies in Frey's book--related to police reports, court records and interviews with law enforcement officials. Frey lied. He lied to his readers--among them, Oprah. Oprah never forgave Frey.
Perhaps unsurprisingly, Frey now writes fiction.
So, how truthful does your memoir have to be? As truthful as you can make it when you write from memory. But, keep in mind one thing: if a reporter can check something out and document it, you'd better do that yourself before you ever commit it to paper and call it nonfiction.