I recently was working on a custom page that required a unique URL. That’s pretty easy to do on WordPress right? Just create a new Page and click on the “Edit” button to change the URL.
I started to scratch my head (more then once) when WordPress would not allow me to edit the URL to the URL that I wanted. After thinking for a bit and scratching my head a few more times I realized that I had a custom post with that same URL in my trash.
So to fix this issue I deleted that page permanently from the trash and lo-and-behold I was able to update the URL permalink. This strategy will probably also work for posts.