RSS <guid> could point to bad URL

[jidanni]

I noticed the <guid>s in RSS Feeds.

I noticed that they sometimes differ that their corresponding <link>s.

OK, they should never change, and indeed will still match valid
urls, if these <guid>s are like ?p=123.

However there is a case that if one chooses one permalink method (via
the admin panels), then publishes an article, then switches to a
different permalink method, the RSS may then contain <guid>s that
will just get 404 responses. Bad?

The guid is not a permalink, not a URL, just a globally unique identifier. Please see #6492

OK, I note some RSSs make GUIDs like "234 at norfsdale.net" so they
don't even look like URIs, so nobody gets the wrong impression...

I agree that the guid could be in a better format, but #6492 is a better place to discuss it.