Twitter -- allow people to edit their tweets and put a check on the levenshtein distance between the edit and the original tweet and make it remain in some value range so that spelling mistakes can be fixed but tweets can't be radically changed.
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Would still allow people to remove/add key words, wouldn't it?
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It certainly could. Although I think a few additional tweaks could nail down something workable. The big issue would be things like slipping in a "'t" or "not" to totally change the meaning of the tweet.