I'm getting this:
One or more of your last posts has failed to send a status update to Twitter. The Tweet has been saved, and you can re-Tweet it at your leisure.
403 Forbidden: The request is understood, but it has been refused by Twitter. Reasons: Too many Tweets in a short time or the same Tweet was submitted twice, among others. Not an error from WP to Twitter.
But, it's not clear why, since none of the suggested reasons is an issue. New article, no tweets recently.
This has been happening frequently, but not always over the last week or two.
WP to Twitter version 2.6.3.