I was doing some work recently generating level 10 logs for bug 4673 with Samba configured as security = user and it was using an smbpasswd file.
I have discovered that Samba 3.0.25a can not use, user's will be unable to connect, entries in an smbpasswd file created with 3.0.24's smbpbasswd if the user's password is greater than 8 characters.
You can reset the password to the same greater than 8 character passwd with 3.0.25a's smbpasswd command and then be able to connect.
In other words, Samba 3.0.25a (I assume 3.0.25 too) breaks backwards compatability with 3.0.24 created smbpasswd files for entries where the user's password is greater than 8 characters.
*** This bug has been marked as a duplicate of 4866 ***