The name rules were only enforced on the server, so the first sign that a name was not allowed was a rejected submission. The field now checks the name as it is typed and names the exact problem - a space, a disallowed character, or a name that does not start with a letter or number - and an information icon beside the label spells out the whole rule.
Repository names are checked as you type
The new Git repository form now explains what a name can contain and flags a bad one before you submit it.