1) someone forgot to say something in the post they just made, but they wanted to add that little bit to the conversation
2) someone posted something, didn't realize it actually posted, and they pressed the "back" button to amend the post.
3) someone posted something in response to one other post in the same thread, the second post is in response to another post in same said thread
Now, while I know that most of the time this is not intentional, I see this far too frequently to consider it ALL accidental. Now, I'm not here to point fingers or anything, and please don't just PM me saying I'm being a meanie or anything. But, I just wanted to let everyone know of a little feature you can use to fix these mistakes in the future (should you find yourself making them)
When you post, you will see a little toolbar at the top before what you typed. Up there, there should be at least 2 buttons:

