Twitter automatically re-shortens all links with their t.co shortener, whether you want it or not. They also automatically convert all hashtags (#hashtag), all Twitter usernames (@username) and all domain names (example.com), and any other clickable item into t.co links. Each of these automatically generated t.co link takes up 23 characters instead of the actual number of text characters. Twitter performs this conversion to t.co links and only then counts the final number of characters in the tweet text. That sometimes causes the tweet length to exceed 140 characters. They reject the tweet even if your pre-conversion text was less than 140 characters.
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