Foolproof

No security system is foolproof.

Nothing is foolproof to a talented fool.

The system is hardly foolproof.

But, and yes there is always a but, the plan seemed foolproof until unforeseen complications arose.

It is impossible to make anything foolproof because fools are so ingenious.

Our plan is foolproof.

Our plan was totally foolproof.

They designed their plan to be foolproof.

We foolproofed the operations.