Twice, adv. Once too often.

Once is happenstance. Twice is coincidence. Three times begins to seem like a message to be ignored at one’s peril.

Once is random. Twice is coincidence. Three times is a pattern.

Once is happenstance but twice is enemy action, and thrice is a fuck-up.

Sure, kid. Look, there's a very old saying in my family: 'Once is happenstance. Twice is coincidence. Three times is another wizard fucking with you.'

This was the second time in his life he’d been on a horse. It was two too many.

Exactly – you said "often" ('frequently') only once.

Most wonderful things quickly become unwonderful if they are repeated too often. Once-in-a-life is often the optimal interval.

Caution once forgotten could be forgotten once too often.

He who worries about calamities suffers them twice over.

He who foresees calamities suffers them twice over.

He who foresees calamities suffers them twice over.

Experience can be merely the repetition of same error often enough.

If one is actually a certain way and yet refuses to acknowledge it when called by that name, one is harmed not once but twice.