we call the friendly url name of an entry the 'slug' ... the call you saw on that error page (bad duncan turned custom errors off to troubleshoot a problem that only repro'd in production) was to get the path for a specific entry... and we can choose to use the slug (producing a URL like this: /forums/Feedback/436601-Yellow-Screen-of-Death/ for this page) or to not (producing a url like this: /forums/Feedback/436601/)
We use non-slug based urls in places where users won't be seeing the url, like links to the pingback/trackback accepting page for a post, or the edit url, etc...