قَالَ ذَٰلِكَ مَا كُنَّا نَبْغِ ۚ فَٱرْتَدَّا عَلَىٰٓ ءَاثَارِهِمَا قَصَصًا
Musa عليه السلامsaid: “That is the place we were looking for,” so they went back retracing their footsteps. [Al Kahf 18: 64]
There is a very subtle lesson which the learned scholars have extracted from this. So after they remembered about the fish, Musa عليه السلام and his companion went back to the place where the fish jumped out. From this the Ulama say that we are reminded that we can go back and rectify our mistake if required. The most important matter is coming back to the truth even if we erred. The objective is the truth and the truth deserves to be followed more than anything else. To err and make a mistake is not the greatest sin, but after realising that we have erred and not to correct it and to remain and insist and persist on the wrong, that is certainly not liked by Allah.

فَوَجَدَا عَبْدًا مِّنْ عِبَادِنَآ ءَاتَيْنَـٰهُ رَحْمَةً مِّنْ عِندِنَا وَعَلَّمْنَـٰهُ مِن لَّدُنَّا عِلْمًا
So they found one of Our servants on whom We had bestowed Mercy from Ourselves and whom We had taught knowledge from Our own presence. [Al Kahf 18: 65]

Allah described Khidr as a slave even though he holds such a high position. This is in fact praise from Allah as Allah also called His Final Messenger, the greatest man to ever walk upon this Earth ﷺ, a slave.
سُبْحَـٰنَ ٱلَّذِىٓ أَسْرَىٰ بِعَبْدِهِۦ لَيْلًا مِّنَ ٱلْمَسْجِدِ ٱلْحَرَامِ إِلَى ٱلْمَسْجِدِ ٱلْأَقْصَا ٱلَّذِى بَـٰرَكْنَا حَوْلَهُۥ لِنُرِيَهُۥ مِنْ ءَايَـٰتِنَآ ۚ إِنَّهُۥ هُوَ ٱلسَّمِيعُ ٱلْبَصِيرُ
Glory be to Him Who took His devotee (Muhammad) one night from Masjid-al-Haram (in Makkah) to Masjid-al-Aqsa (in Jerusalem), whose vicinity We have blessed, so that We may show him some of Our signs: surely He is the One Who is the Hearer, the Observer. [Al Israa 17: 1]

Allah describes the fact that He had granted special knowledge to Al-Khidr (عَلَّمْنَاهُ مِن لَّدُنَّا عِلْمًا) as a special mercy from Himself towards him (آتَيْنَاهُ رَحْمَةً مِنْ عِندِنَا). This is very interesting, the fact that knowledge from Allah is actually a manifestation of His special mercy.

This is because, in the current age at least, we tend to staunchly believe that education and knowledge (of any kind, even practical training) is restricted inside schools, institutes, colleges and universities. Whereas here was Al-Khidr, who had been taught special knowledge directly from Allah, which he was about to impart to Musa عليه السلام (to obtain which, Musa عليه السلام had intended to go on searching for Al-Khidr, even if it took him years and years to find him), and this knowledge did not require a classroom setting to be learned/acquired.

The kind of learning Prophet Musa عليه السلام wanted to undertake did not involve classes, with a teacher sitting before his or her students giving talks or lectures on a subject, but rather, it consisted of practical “field work” or on-the-job apprenticeship, as they are known in the modern age, in which learning took place mostly via observation.