1) Always effective 2) Guaranteed 3) Guaranteed against failure 4) Impervious to incompetence 5) Infallible 6) Involving no risk 7) Like a perfect plan 8) Not liable to failure 9) Safe 10) Sure to work 11) Surefire 12) To make foolproof 13) Unfailing 1) Failsafe 2) Goofproof 3) Infallible 4) Perfect 5) Sure 6) Surefire 7) Unfailing 
The FoolProof Initiative is a project by America’s credit unions that teaches financial literacy to consumers in the United States and Canada through a variety of programs. It uses video and audio, games and music in a modular system to teach complex as well as basic financial concepts such as understanding credit (finance) and credit score, get...
incapable of failure or error 
[adj] - not liable to failure
foolproof 1. Designed to continue working properly in the face of any kind of human error, incompetence, or misuse. 2. So well thought out that failure is thought to be impossible.
unfailing adjective not liable to failure; `a foolproof identification system`; `the unfailing sign of an amateur`; `an unfailing test`
involving no risk or harm, even when tampered with. · never-failing: a foolproof method.