If you forgot HaMelech HaKadosh · הַמֶּלֶךְ הַקָּדוֹשׁ
If you mistakenly concluded “HaKel HaKadosh” [the regular-year wording] and catch it within the time it takes to say a short phrase, then say “HaMelech HaKadosh” — it's valid. If not caught in time, you must return to the beginning of the Amidah; the same applies if you're unsure which wording you said. During the rest of the year, saying “HaMelech HaKadosh” by mistake does not require going back.