There used to be a rather negligent backdoor in some 3COM products - you could log in with username 'debug' and password 'synnet' to get a full admin shell with

There used to be a rather negligent backdoor in some 3COM products - you could log in with username 'debug' and password 'synnet' to get a full admin shell with extra debugging commands. IIRC it wasn't printed in the user manuals, but was fairly obvious from poking around in the firmware updates. Fortunately the password could be changed, so it wasn't a completely irreparable security hole.