[MikaN]

Bug:

If you have Forum Component URL setting active (under Settings ->Site Name / URL / Contact Details) it will be added to the bit.ly url.

For example: if you have there http://www.example.com/forum

then every twitter link is broken because bit.ly link is wrong. It looks like this:
http://www.example.com/forum/http://www.example.com/showthread.php...

The reason for this is that if that Forum Component URL is set that
'fetch_seo_url('thread', $threadinfo);' function will return

http://www.example.com/forum/showthread.php

but if it's not set it will return

showthread.php

So when you have this in your code:
$baseURL = $vbulletin->options['bburl'] . '/' . fetch_seo_url('thread', $threadinfo);

it means that if Forum Component URL is set it goes like this:
$baseURL = www.example.com/forum . '/' . http://www.example.com/forum/showthread.php..
-> $baseurl = http://www.example.com/forum/http://www.example.com/forum/showthread.php

If it is not set it will be correct
$baseURL = www.example.com/forum . '/' . showthread.php..
-> www.example.com/forum/showthread.php...