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Near the top, he was beginning to realize his mistake.

Perhaps he'd even let her go once he realized his mistake.

By the time he realized his mistake it was too late.

She soon realized her mistake, but it was too late.

By the time security realized their mistake, it was too late.

But by the time she realized her mistake, it was too late.

We did not realize our mistake until it was too late.

It did not take him long to realize his mistake.

It was the wrong thing to say; he realized his mistake at once.

But he had realized his mistake a few moments too late.