Haven't ever seen this before. Tried lots of different searches but couldn't find anything, so hoping that the WPMU heros may know what the issue is.
Basically, I'm not seeing "this slug is already in use" errors when creating new posts or pages with slugs that have already been used.
So we have "inspiration" as a page and then someone created "inspiration" as a post, but WP didn't automatically slug it with "inspiration-2" as is the norm.
Any ideas what could be interfering with this? Definitely not a plugin as I've disabled all of those and the problem persists.