When your followers retweet you, they add six characters plus the characters of your username to the front of the tweet:
RT space @[username]: space. If you want the retweet to fit the parameters of Twitter, your usable characters are reduced by however many characters that space takes up. In my case, lucky number 13. So my tweets that I want/hope to see retweeted must be 127 characters.
I see organizations make this mistake more often than people, who typically don’t care about RTs unless they’re super-addicted to Favstar or Klout.