Why does my Mac (OS X 10.9) try to log into our fileserver as GUEST?
The two places that come immediately to mind are the favorites section of Go to Server... so you might clear that out. Secondly, look in the Login Items pane of Users &amp; Groups preference pane.

Also, if more than one user logs in to the Mac, be sure to eject the share before fast user switching (or logging out all users but one) to prevent their mount from controlling the access.

Apple is in the process of switching from AFP to SMB2 as the default share - so perhaps some changes there are not completely done or documented.