I'm guilty of doing stuff like this.. When I write a node, even though I may preview it and it may look fine, I always go back and re-read through some of my posts (esp. if they're garnering lots of rep), and frequently change some wording slightly, clean up code occasionally, and sometimes add to it.
Though when I am making a significant content update, I will make it obvious with an "Update:" stamp. Unless I make the update within a few seconds after I post it (which, alas, I also do frequently).
I do try to take heed to replies, though, beforehand, so I don't change things that invalidate someone's reply and make them look stupid. That's just rude.
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