I've made a stored procedure stp_LogError which will be used to have a
central errorhandling system.
(So in the usermade stored procedures in the catch-block stp_Logerror will
be used to log the error).
I've granted execute permissions for stp_LogError to the public role.
Afterward I added a NT-login to the server and added this login to the role
public in the database.
I granted this user execute rights to a stored procedure in which
stp_Logerror is used.
When this user calls this stored procedure, this stored procedure executes,
but when an error occurs we receive an execute permission denied on
stp_LogError for this user.

When I check the login for this user it is made a user in the database
through the role public.
When I check the role public in the database  the user is not a member of
the role public.

I thaught that every user which is granted acces to a database is member of
the public role, so when i granted execute rights for stp_logerror to the
public role, everyone (except the explicitly denied ones) would have execute
rights on stp_LogError.

Can someone explain this to me