If you delete a user that has previously created or edited content, and then access that content, you will get an error in the "Details" tab in the "Creator" and "Last contributor" fields:

Creator -> An error occurred while loading the creator.
Last contributor -> An error occurred while loading the creator.

To verify this, do the following:

1. Prepare new 16.04 (1.3) installation;
2. Login to the admin interface with an user with sufficient permissions (e.g. administrator);
3. Create a new user (for this test, created an editor);
4. Logout, and then login as the new user;
5. Create a new content (for this test, created an article);
6. Logout, and login as administrator (or another user with sufficient permissions);
7. Delete user created on step 3;
8. On the "content structure" section, go to the content created on step 5, and then to the "Details" tab. The "Creator" and "Last contributor" fields will display an error:

Creator -> An error occurred while loading the creator.
Last contributor -> An error occurred while loading the creator.

This should be handled in a more user-friendly manner.