I'm sure anyone who has spent any time on the forums has run into this situation. Someone writes something particularly nasty in order to intentionally anger one or more people who are posting on a thread. They do this intentionally in order to try and anger the other person enough to get them to then write something harsh back. The troll then immediately responds by crying fowl and quoting the retalitory remark and how mean it was.
The troll then sneakily goes back and either removes whatever it was they said to begin with that was so inflamatory or they replace it with something harmless and reasonable. Thus leaving only the angry response of the person they were trolling pluss they ensure that the person can't remove the remark which now appears completely out of context because they have quoted in their later post.
My solution to this, although only partial, is to at least clearly time stamp a post when it has been edited. This way there is at least evidence that conversation has been manipulated after the fact.