infallible
1. [ adjective ] incapable of failure or error
Examples:

"an infallible antidote" "an infallible memory" "the Catholic Church considers the Pope infallible" "no doctor is infallible"

Related terms: fallible inerrable foolproof
Similar spelling:   inapplicable
  invaluable
  infallibility
  in_full
  inflexible
  inability
  in_flight