Bug#700558: I think I found where that button is coming from

[Praveen A]

Hi Josselin,

Can you install krb5-auth-dialogue package and see if you can
reproduce the issue? As soon as I login a notification box comes up
saying your kerberos credentials have expired. This box is waiting to
be closed but only a part of close button is visible. Sometimes
closing this notification causes the whole gnome-shell to crash as
well.

Praveen