Earlier quoted context omitted.
Twitter doesn't pay attention to URL length any more. They will force all URLs through t.co even if that makes them longer.
I've found that it doesn't like to shorten certain things, though I haven't quite nailed down what "certain things" are. I tried linking a recent blog post I'd done, which is at least 30 characters (along the lines of http://website/really-neat-post-title ), and when I pasted or typed it into the tweet, it wouldn't autoshorten and would consume those characters. I find myself using goo.gl a distressing amount of time…
As an experiment some time ago I grabbed a random sampling of URLs from twitter's limited firehose - and found that on average they spread the user's trail across no fewer than 4 redirects before the final destination was reached. Some as many as 7 or 8.
I've stopped posting URLs to twitter, out of respect for my followers.