قُلْ هَلْ نُنَبِّئُكُم بِٱلْأَخْسَرِينَ أَعْمَٰلًا · ٱلَّذِينَ ضَلَّ سَعْيُهُمْ فِى ٱلْحَيَوٰةِ ٱلدُّنْيَا وَهُمْ يَحْسَبُونَ أَنَّهُمْ يُحْسِنُونَ صُنْعًا
"Say: 'Shall We tell you of those who lose most by their deeds? They are those whose whole effort in the life of this world has gone astray, while they suppose they are doing good work.'"
The structure of the sentence must be recorded: two states are reported at once — ḍalla saʿyuhum (their effort went astray) and yaḥsabūna annahum yuḥsinūna ṣunʿā (they suppose they are doing well).
I record this as my own reading: the verse diagnoses not ill intent but a wrong measure. The person works, gives his labour, and is content. The only thing lacking is knowing where the effort is going.
And this agrees word for word with the sixty-eighth verse of the sūra: wa kayfa taṣbiru ʿalā mā lam tuḥiṭ bihī khubrā. There the lack of knowledge produced impatience; here it produces a false confidence.
In Fuṣṣilat at 41/23 (wa dhālikum ẓannukum … ardākum) and in al-Jāthiya at 45/24 it was treated that conjecture is a cause of destruction. I rely on that treatment.
And the verb yaḥsabūn had occurred in the ninth verse of the sūra as well (am ḥasibta) and in the eighteenth (wa taḥsabuhum ayqāẓan wa hum ruqūd). These three occurrences: one belonging to the reader, one to the scene, one to themselves. This is a pattern that can be verified within the sūra.