If a character in a URL can change, it will.I'm not the only person to think this but I just wanted to get that thought out in simple, direct language.
But what do I mean? Take Worldcat URLs such as http://www.worldcat.org/oclc/502674170. That "www." is annoying and should not be part of the URL that Worldcat presents as its permanent identifier for the book. At some point in the future, somebody there will realize this and remove those unnecessary characters. But http://worldcat.org/oclc/502674170? Now you're talking! And look, it already works.
It's true that the "oclc" could be shortened so maybe I need to qualify the formulation, but I'm not going to for the following reason. Changing those characters would risk collision with other identifying schemes that Worldcat supports such as http://worldcat.org/isbn/0754677737 . The 'www.' is unstable because it can be removed without breaking anything.
The simple formulation stands: If a character in a URL can change, it will.
The implication is, "be aggressive about removing all unnecessary characters from your URLs." The following is a horror-show:
If a URL looks unstable, it is.