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?
Fake Qurans are hard to fool anyone with because plenty of people (me included, an atheist) memorized bits of it verbatim. People can easily cross reference anything with the thousands of online websites that offer digital Quran copies. Many have tried, and many times it eas accidental typos that urged authorities to draw back and destroy many copies with "errors". There is also this one wacko cult leader obsessed with numerology and the number 19 who republished the Quran with some added verses. It was not taken lightly.