on the registration page for the membership site plugin, a user is allowed to enter a username with a space in it. The registration will go through, but wordpress in the background will convert the spaces to a dash. The user will have no idea that wordpress added the dash. I think that the registration form should reject a space in the username, and tell them in the returned error that they can use a dash or underscore if they wish. Yes the user receives an e-mail with there username and password, but they are not going to read that, and if I was unaware that I should not use a space, i would choose something different, than now have to remember for this site only, i now have to add a dash instead of a space.