foolproof

“Those can be defeated but that can take a lot of time,” Sutter said.”It’s not foolproof but at least it’s a strong deterrent.”

Both the bunker and the 20-minute red cards are very solid concepts in theory, but in practice they are not foolproof.

Her plan to get back to Ashton Place and the Incorrigible children was in no way foolproof.

It was because her plan about the séance, which had seemed so simple and foolproof when she first thought of it, now appeared to have more holes in it than a mole-infested garden.

It was hardly foolproof; indeed, it was almost certain to fail more often than it succeeded.