The mistakes’ author writes while pointing out a grammatical error in The Pilgrimage:
22/19: ‘ikhtasamuu should be ‘ikhtasamaa.
The verse is:
هذانِ خَصْمانِ اخْتَصَمُوا فِي رَبِّهِمْ فَالَّذِينَ كَفَرُوا قُطِّعَتْ لَهُمْ ثِيابٌ مِنْ نارٍ يُصَبُّ مِنْ فَوْقِ رُؤُسِهِمُ الْحَمِيمُ (19)
The author maintains that اختصما should have been used instead of اخْتَصَمُوا. Apparently, it looks right but as always the author is unacquainted with the style of Arabic language that how it changes pronouns and tenses. For example, in the last verse six groups of people were being discussed. The verse is:
إِنَّ الَّذِينَ آمَنُوا وَالَّذِينَ هَادُوا وَالصَّابِئِينَ وَالنَّصَارَىٰ وَالْمَجُوسَ وَالَّذِينَ أَشْرَكُوا إِنَّ اللَّهَ يَفْصِلُ بَيْنَهُمْ يَوْمَ الْقِيَامَةِ إِنَّ اللَّهَ عَلَىٰ كُلِّ شَيْءٍ شَهِيدٌ (17)
The Believers, the Jews, the Sabians, the Nazarene, the Magians and the Polytheists of Arab formed six groups. Five out of these six were groups of non-Muslims. Together they stood against Islam. Due to this two wider groups formed i) Muslims and ii) non-Muslims which were hostile to each other. In a situation like this that duality refers to two groups instead of two individuals, to use a plural tense is sometimes more eloquent. We call it that the pronouns have been used according to semantics (ضمیر علی المعنی). That is, when The Speaker said خصمان hundreds of people were in His mind. Therefore, He used plural tense elsewhere. This brings the text closer to real situation i.e. the mind of the reader is not directed towards two individuals but towards two groups consisting of many people. Not doing so would have affected the eloquence of the text.