There use to be a site called batcave.net that gave free space and a subdomain. I had the name search.. search.batcave.net until one day when they decided to take back the search subdomain name and use it for themselves. From that point I've avoid great common usernames that one day might be too great
It's the exact reason that GitHub moved to github.io for GitHub Pages hosting. At first there were a few individual users still grandfathered in to be allowed to use <username>.github.com for their GitHub Pages, but I don't remember who they were and I don't know if that still works for them this many years later.
The real reason is avoiding user-generated content from stealing authentication cookies. If worldmaker.github.com can run a little bit of javascript to add @worldmaker as admin to all the user's repositories on github.com, well, that's a problem.
Both reasons are true. Also, relatedly, "stealing perceived authenticity": if a user sets up "help.github.com" or "about.github.com" or "wwww.github.com" and then runs a scam from it, it looks like GitHub is running the scam.
We had a similar problem when I worked at Belgacom Skynet from 1998 to 2001. We didn't separate our customer user aliases from our employee user aliases -- customer and employees alike were all "user@skynet.be".
The customer who had previously owned "brad@skynet.be" was real pissed when I got hired and his alias went away. And I don't blame him. But there wasn't anything I could do about it.
This mixing of employee and customer addresses on the same server really bit the company hard one day when a certain VP of the company felt that he had to mail out a 50MB PowerPoint presentation to every single employee in the company -- all 500+ of us under the alias all@skynet.be.
And no, at that time, the mail server did not have 25GB of disk space for the mail queue. He broke all mail across the company and all customers, until I went in and fixed his mess. Allowed mail message sizes went down to 5MB, and all mailing list type aliases went away and were replaced with a separate mailing list server. That VP was super pissed that he couldn't send out 50MB PowerPoint presentations any more, but I didn't suffer any repercussions due to what I did to fix the mess he had created.
Soon thereafter we bought new servers to use for customer-only traffic (matched pair of Sun 420Rs with a dishwasher-size external RAID array that they were connected to over fiberchannel), and we kept the old mail server (Sun E250 with eight internal hard drives with software RAID) for employee purposes.
