1. Failing to Document Activities,
2. Not Admitting They Made Changes When Something Broke,
3. Read User Mails or Go Through User Files,
4. Look at Someone Typing Their Password,
5. Wait Until Tomorrow,
6. Start a Major Change on Friday,
7. Allow Someone Non-Technical to Access the Admin Account.

Via https://www.reddit.com/r/sysadmin/comments/4sfye4/things_a_sysadmin_should_never_do/