A client has reported that since last Friday it's no longer possible to edit Tweet text if it's been specified in an AddThis "title" property. What happens is that the user edits the text displayed in Twitter's edit popup, but when they click the Tweet button the text that goes out is the original title.
You can see this happen from your own site in the example: http://support.addthis.com/customer/portal/articles/381242-url-title#example
In that example the text starts as "An Example Title". If you select Share->Twitter then edit the text in the Twitter popup and send the tweet, what is actually tweeted is still "An Example Title".