The mistakes’ author writes while pointing out a grammar error in The Prophets:

21/3: ‘asarru should be ‘assarra

i.e. أَسَرُّوا should be أَسَرُّ in the following verse:

لَاهِيَةً قُلُوبُهُمْ وَ أَسَرُّوا النَّجْوَى الَّذِينَ ظَلَمُوا هَلْ هَٰذَا إِلَّا بَشَرٌ مِثْلُكُمْ أَفَتَأْتُونَ السِّحْرَ وَأَنْتُمْ تُبْصِرُونَ

The rule is that when subject follows the verb then we have to use singular verb form. But here the subject does not follow verb because it was already there in the text in the form of هُمْ يَلْعَبُونَ etc. الَّذِينَ ظَلَمُوا is just a apposition for the subject pronoun of أَسَرُّوا. 

Before pointing out this as a mistake the author should have consulted a few classic exegeses to save himself from embarrassment.