What's the most severe example of a fake version of a book you've ever seen?

When a book becomes influential enough, someone might try to impersonate it, since publishing doesn't follow any hard rules.

For example, I was explaining to someone that, after (surprisingly not before) I got a job at my local library, I took out a communist manifesto, which I later learned was a fake, with writings in there that were not consistent with the official communist manifesto, such as a call for free love.

I have also spotted a lot of fake versions of Mark Twain books come in, which has a lot of parts deleted or inserted based on the writer's desire.

On the other side of the issue, lately I've been watching a lot of the events unfold in the middle East and have wondered why nobody just ends violence over there for good by making fake Qurans. One or two people have hinted they've tried, with some altered movements centered around it (would you call this government gnosticism), but it's not something you always hear.

What's the most severe example of a fake version of a book you've ever seen/encountered?