It proves that there is an issue on login when you want to change your username.
If you change your username and hit enter without completing the captcha, it will change your username and no error about the captcha not being completed.
After that, you won't be able to fix your username before 7 days.
It should be like that (solution) : if you don't complete the captcha, it won't change your username but will return an error and tell you to complete it before having it changed.
Someone reported the issue here : https://forum.xat.com/topic/3258-username-change-no-actual-change-captcha-incorrect/
Proof of the concept : http://i.imgur.com/J6EyVpI.gif