Real usability starts when one and only one URL is used for each piece of content, and all hyperlinks on the site point to exactly that URL. If that piece of content must respond at a different legacy URL, it's 301 redirected to the new one (but no autogenerated content on the site should point to the legacy URL).
This includes character encoding in URLs. For example, "-", "%2d" and "%2D" may mean the same to the server, but make URLs different from client's standpoint (browser history, search engine, etc.); only one encoding for every character should be therefore used.
I hope I'm not getting too hard here, but this is what consistency of URLs really means. If you've visited a page but see a link to it in blue (unvisited) color on another page, you know that the site is not being consistent. Some sites hide it by setting the link colors to the same color :)
