Which of the mentioned cases of forgetting the middle Tashahhud necessitates prostration of forgetfulness?

If someone stands up to the third rak'ah inadvertently, forgetting the middle tashahhud, and remembers only after having fully stood up or begun reciting, then he must continue his prayer until he completes it, then perform two prostrations of forgetfulness (sujood as-sahw) before the tasleem. This is based on the Prophet's (peace and blessings be upon him) saying: "If one of you remembers before he has fully stood up, he should sit. But if he has fully stood up, he should not sit."