Bungled an attempt to add a login to my account recently and created a sock instead. Decided not to give myself sweet sockpuppet rep, so I tried sending in a merge request.
You can see from the image that the validation for the profile URL is a bit overzealous, and won't accept a URL without the slug on the end (in my case, /will).
Mayyyyybe it's a feature, but I'm thinking it's a bug.