More often than not, most people type website/blog addresses without http or www. Nowadays, this is not much of a problem, as most sites have 301 redirects from non-www version to www version.
Whenever I have linked out to other blogs, I have used the URL from the address-bar. This avoids any typo in the URL. However, when I linked to the official Orkut blog on this post, I forgot to add http://.
And WordPress made me pay dearly. Apparently, WordPress adds the host domain(which, in this case is techzilo.com/) before the URL, meaning a useless jumbo like http://techzilo.com/en.blog.orkut.com.
This creates an unreachable link, which adds to the frustration of the readers. Some time back, I had a barrage of comments about the unreachable download URL, because of the above-mentioned error.
However, this is not just about reader frustration – the broken link can have serious implications on your search rankings. Your PR may (and probably will) be the first to suffer.