Fielding's dissertation was based on looking at the design principles that make the web work well and this is one of the things that, at a high level, most websites get right. If you did something like create sequential pages e.g.
/foo/bar/1 and then reassigned them randomly then that would violate this but I doubt anyone does this, at least not for purposefully or for very long. I propose the reason for that is because URI stability is so fundamental to making things work and Fielding is simply recognizing this.
The only thing that I can think of that I've seen on a regular basis is a website reorganizing all their URIs such that old links to them return 404. It's not that the pages are no longer available (a.k.a 410: GONE), they just dropped the old URLs without forwarding. A big blue company did this with their online documentation about a decade or so ago. I would regularly find a thread related to a problem we were having and someone posting a response like "the solution can be found here" with a link that just gives a 404. Thankfully, I no longer have to care. Fielding is right, this is intolerable.
While this might fall into a different category of problems, I argue that it is an example of this because an existing URI's semantic meaning has changed from something to nothing.