If a user signs up and uses a space in their user name e.g. "john smith" and then forgets their password and requests a password reset. they cannot reset the password.

They get an error message "user 'john' not found"

Users should be prevented from using spaces and other white space and non printable characters in their user name.

Changing a user name should also be a heck of a lot easier, this page which outlines the process is a nightmare
http://confluence.atlassian.com/display/DOC/Changing+Usernames

the workaround is for admin to set their password for them.