The URL http://qntm.org contains 15 characters. Twitter's naïve URL "shortener" helpfully shortens this to 20 characters, leaving me with 120 remaining when I should have 125:
If I type a full 140-character string, Twitter reports that I am 5 characters over budget and refuses to let me tweet:
When is a URL shortener not a URL shortener? When it fails to ensure that the new URL is shorter than the original.
I'm a professional software tester, you know. We catch this kind of thing.
This was the case as of 2011-08-16, and I only saw it in the main web interface, not when you (for example) click the "tweet this" button at the bottom of this page. Once it's fixed (I'll let them know about it tomorrow and I expect it'll be rectified in a day or two) I'll update this page, but until that time, try it for yourself.