An illustration: You spend time composing a comment to a thread, say 2 minutes. Then, when you hit the Previewbutton (which you must hit before committing the comment) you realize that another comment has been made in the time it took for you to compose your post that, for example, says what you have just written, or raises an interesting point you want to respond to. Rather than re-writing the entire post, you add 'On preview, I agree with ZenMaster' so people don't wonder why you missed that key point when your post comes sequentially after his.
(adapted from [cell divide's comment] in [a thread about On Preview])
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