Lengthy URLs are shortened - notice the .... in the text between the beginning part of the URL and the end part?
I think they've literally copy and pasted that, rather than the link itself.
e.g. here's a long link
http://xenmediagallery.com/threads/...ally-really-really-long.1069/page-2#post-5912
I think it's just unfortunate the positioning of the ellipsis in the URL text. When that is copied it breaks the URL. In some cases the URL might not break and stay valid.
Anyway, the safest way to copy and paste a link is to right click the link and select "Copy Link Address" from the menu.
When a valid URL is pasted into any content it should be automatically linked.