In the "Create an account" page (after authenticating), it isn't very clear to users that spaces are not allowed in usernames, nor what to do about that.
The notice reads:
Usernames must contain only numbers, letters, period, underscore and hyphen, and can not end with a period. They must have no more than 64 characters.
I suggest changing this to:
Usernames must contain only numbers, Latin letters, period, underscore and hyphen, and can not end with a period. Spaces should be replaced with an underscore (_). They must have no more than 64 characters.
(Bolding just for visibility of change. Including an actual underscore so that users can copy&paste it. Specifying "Latin letters" per docs.)
I just helped an editor through this confusion, and I didn't realize myself that spaces weren't allowed which added to the confusion (I think I assumed that Phab automagically replaced them with underscores, like browsers do with typed URLs).