This is really a UX question, not design per se.
https://gerrit.wikimedia.org/r/#/c/104926/ (an under-review change) adds a warning if you choose a username that needs to be reformatted/canonicalized. For example, "my username" is changed to "My username".
It displays the warning, puts the new username ("My username") in the box, then you have to retype your password (twice), then press enter to confirm.
The main potential benefit, as I see it, is that if you don't like "My username" (the reformatted version), you can choose a different username entirely.
User experience feedback would be welcome; you can post on the change or here.
Matt Flaschen