Cases when user dissociation fail

What are the possible cases when user dissociation will fail?

User dissociation can be done via the Edit Customer Preferences form, Edit Customer Group form, Edit User form. The following list the possible use cases when user dissociation will fail:

Use Case
Troubleshooting Tip
If the user is the account contact
Log in to Site24x7 and go to Admin > My Account > change the account contact.
If the user is the last contact of an User Alert Group
Delete the User Alert Group or remove the user from that User Alert Group

Both the above troubleshooting cases have to be done in that particular customer account only.