Oh, and one more 7777 - to avoid in the US at least:

§ 7777: § military interception (US) ("Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.")

Transponder Codes

"DOD interceptor aircraft on active air-defense missions and operating without ATC clearance in accordance with FAA Order 7610.4"

https://en.wikipedia.org/wiki/List_of_transponder_codes

David