* If you go to "Appearance" and try to change your service tag to "I" or "L" followed by "17" (for example I17 and L17), it will result in a message that they are reserved by the UNSC (Master Chief's number is 117).
Also anything that ends with "00" will result in the same message. Entering "T17" and "A55" will result in a message stating that the service tag is not allowed under current UNSC regulations.
* If you try to use "F46" as your service tag you will be told that it cannot be used under UNCS regulations and rules. It is interpreted as an offensive three letter word for same sex relations.
Enjoy!