sudoers disaster

[R.L. Reingard]

Hello all,
i guess i did make a great mistake.
cause of the ongoing discussion on Root/Sudo/Passwords, i made an other  
user, put this new user into the group 'Admin' and tested if this new user  
can use sudo with its own password. test was okay.
then i took the default user (which had sudo rights) out of the group  
'Admin' and tested if the default user cannot anymore use sudo. test  
passed.
then i switched to the new user AND ... now a sudo with the new users  
password FAILED.
no a have two user accounts AND none of them has sudoers rights.
what the hack is that?
please help,
René