I found posting using XMLRPC (in my case the WordPress iOS client) there is apparent bug. I am also using bit.ly shortener.
The bug manifests when the title of the post is over-sized. An error appears showing "403 Forbidden: The request is understood, but it has been refused. This code is used when requests are understood, but are denied by Twitter. Reasons can include: Too many Tweets created in a short time or the same Tweet was submitted twice in a row, among others. This is not an error by WP to Twitter."
But I think this is a problem with the WP to Twitter plugin. After a lot of playing about, I have fixed it by patching my local install:
I changed line 437:
$diff = $url_strlen - 19;
$diff = $url_strlen - 20;
Hope this helps someone, can can be tackled in the next release of WP to Twitter.
Love the plugin, great work!