Today during some live testing, I got two complaints from two different users that they could not set up their accounts.
Both times, the women were trying to create user names with a space. For example:
Lots of texts later I discovered their errors. Be sure to alert your users to adhere to conventional user names. Some sites don't require it, and they get used to 'what other sites do'. And if you're trouble shooting with a customer, ask them first, what they're using as a username.