We only need to be lucky one time. You need to be lucky all the time.

The line is used to describe asymmetric risk between an attacker and a defender (e.g., criminals vs. police, assassins vs. targets): the attacker can fail repeatedly and still succeed if they get one successful attempt, while the defender must succeed every time to avoid harm.

It highlights how repeated attempts favor the side that only needs a single success, emphasizing persistence and the compounding advantage of many tries against an opponent who must maintain a perfect record.

Poachers have to be lucky all of the time. We only have to be lucky once to catch them.

We have only to be lucky once; [Thatcher/you] must be lucky all the time.

With a book you have to be lucky once in a while. With flying you have to be lucky all the time.