We have noticed that it is possible to register via checkout, allowing a username either having spaces in it (okay but problematic), or being only spaces (bad).
We recommend the following:
1) Do not allow a username to have any spaces before or after. Or just automatically remove the beginning and trailing spaces before submitting (but then you'd have to check if what's left, already exists as a username)
2) Definitely do not allow a username to be spaces only.
3) Some option if spaces are allowed at all, would be welcome.
Thanks