This is now fixed. If the Sign In button is used then if the user/Display name does not exist then the page is cleared and refreshed.

An example of an annotation on a Kasten page.

This is an example of a referred note that no longer exists. Should they disappear? (Which means a periodic crawl to see if URLs still point somewhere) Or is the record of a deleted note (in this case a Tweet) a useful thing? It will vary, case on case.

This isn't picked up as a URL but as what the URL is pointing at, kinda. Which would be fine but it isn't a URL but a file name.

This is a Twitter bug. In the body of the Tweet the shortened 'URL' is used. In the array of URL dictionary entries it's the original file name/URL.

Because t.co URLs are not permalinks the actual URL array is fine, but if the text is used on its own it may or may not go anywhere.