If you have a set an user profile filed as a dropdown menu and not set an option for a empty string and you try to create an user account whith the function "create_user_record" in moodlelib.php the following error will show up:

{{Debug info: Column 'data' cannot be null
INSERT INTO mdl_user_info_data (userid,fieldid,data) VALUES(?,?,?)
[array (
0 => 8356,
1 => '8',
2 => NULL,
)]
Error code: dmlwriteexception }}

This happens per instance, if you log in the first time with a shibboleth authentification.