The mistakes’ author maintains that in Taha (20:63) هذین should have been used instead of هذان. The verse with highlighted ‘mistake’ is:
قَالُوا إِنْ هَذَانِ لَسَاحِرَانِ يُرِيدَانِ أَنْ يُخْرِجَاكُمْ مِنْ أَرْضِكُمْ بِسِحْرِهِمَا وَيَذْهَبَا بِطَرِيقَتِكُمُ الْمُثْلَى
His own words are:
20/63: haazaani should be haazayn
This, too, is a special style of Arabic language. For demonstrative adjectives like هذا there is a rule that sometimes according to the context the dual noun of هذا is treated just like هذا itself. Hence, i‘rāb (nominal and adjectival suffixes) do not appear on it (in the same way they don’t on هذا). (See I‘rāb-ul-Qur‘ān by Abu Jafar An-Nahas vol. III page 32). As a benefit the meanings of ضمير الشأن أو القصة are introduced and the sentence becomes more eloquent. In this case the translation will be: “They said: Look! the fact of the matter is that both of these two wizards…”
So, we request the mistake finder to improve his knowledge!