A while ago Twitter allowed tweets that were longer than the original 140 characters. Some bits of a tweet no longer counted towards the character count and they doubled it to 280 characters. To prevent breaking legacy applications they require an additional parameter when interacting with the API. You can find more details here
It took me a while to update my own site to use the new API response, with the full_text, but I finally did that. The main thing was modifying the database to cope with the longer response. I then noticed I wasn't performing a Unicode regular expression, which broke a recent hashtag, which I've now fixed.