Seen on Message Boards and blogs and the like, the Ninja Editor is the type of person who makes a post, then almost immediately goes back to and then edits it quickly and without comment — like a ninja. *cue rain of cherry blossom leaves*
This is usually innocent (like fixing typos, in which case it's customary to add something like "Ninja-edited for typo" just for clarity) but can also be used as flamebait, when a Ninja Editor's edit radically changes the content of the post. This can lead to an even more confusing situation where the earlier replies to a Ninja Edited post are replies to the original unedited post and later replies are replies to the new, edited post.
In the case of a total-post change, it may be deliberate trolling in order to bring about maximum confusion, or may be a newbie recanting after inadvertently running afoul of Internet Backdraft, in which case it may be extremely difficult for late-comers to the thread in question to figure out what exactly went the hell on in the first place. A more malicious application might be to ask a question, then once a few replies have been posted, edit the original post so that the answers now appear to be incriminating (i.e.: getting anyone to respond with a number under 13, then changing the question to "How old are you", resulting in an immediate ban under minimum age rules. This is why GameFAQs didn't allow editing for a long time, and while it is now allowed the above actions will get you banned.) Or even worse, make an offensive comment that tricks someone into angrily replying back at them, only to delete the post they made so that the person's angry reply is now orphaned and it looks like they made the offending comment. For this reason, the lion's share of these messaging application developers have made it so that replies to posts form parent-child trees, and if the original post is deleted, the replies also go away so that no one can be caught in this fiendish trap.
This can get really, really annoying in certain communities (especially LiveJournal) where you can now edit comments, as most people have settings that email the comments to them, so in the space of a few minutes you can have half a dozen of practically the same comment clogging up your inbox.
People that are afraid of this happening to a significant comment may quote them so that it's part of their quote. This ignores that quotes can also be altered. (Usually just by moderators, however.)
This is the reason that some fora allow editing of articles only at certain times (let's say, one hour) after the post was made. Others only allowed editing until someone responds to the comment. More than a few popular web forum packages will, after a certain arbitrary boundary is passed (after X minutes, Y edits, edits after another user has posted, or in some cases the very first edit, no matter what) mention, in smaller text, that the post has been edited.
A somewhat related phenomenon is the Ninja Post, where during the time an entirely new message is being written or uploaded, one or more other corespondents will submit messages into the conversation where the aforementioned one was intended to go. In this case, specific references to location or primacy (e.g.: Like the last post says , I'm surprised nobody's mentioned ) will be rendered confusing or misleading. This is one of the disadvantages of having a slower internet connection. A derivative of this takes a memetic form known as the Combo Breaker, where someone on a message board tries to post a series of images in sequence that form a larger image when pieced together, but someone cuts in with a post that interrupts the sequence. This is especially rampant on 4chan in attempts to taunt some for being too slow on the draw, and some users have even taken to making custom images that portray them destroying the offending combo breaker to preserve their collage.