There are many secrets in Mahabharata, as I have read almost all books (Adi parva, Sabha Parva, Vana Parva, Virat Parva, Udhyog Parva, Bhishma Parva, Drona Parva, Karna Parva, Shalya Parva, Suptika Parva, Stri Parva)
1)Sahadeva knew everything about the future and was a great astrologer. He knew Shakuni’s motive in driving Duryodhana to war against the Pandavas. He knew that Shakuni was seeking revenge for his sister’s marriage to a blind man. He was supposed to have known the events of the Mahabharata War beforehand.
Alas! He still kept silent because he was under a curse that if he revealed anything to anybody he would die.
This undoubtedly remains the best kept secret of the whole epic, because had he not kept his mouth shut, we would have been talking about a completely different story today.
2) Arjuna has many mass destruction weapons from Shiva, Indra (including his favourite Varja), Brahma, Narayan, Yama, Varun and from almost all Demi gods. He didn’t used those weapons (used some of them when required and to counter enemy weapons). If he would have used, war would had finished much earlier, but as it was Dharma war for him and Krishna, they didn’t use mass destruction weapons. Arjuna fought very mildly with Bhishma due to affection toward him and Krishna need to warn him twice to be aggressive (once by picking chariot wheel and once by sudarshan chakra) against him, and Bhishma also fight mildly against Padava brothers (he used to kill other allies kings of Pandava and soldiers).
3) Arjuna has token an oath to not to counter any ‘Narayan’ weapons (Why? I don’t know.)
4) In some other edition of Mahabharata (BORI Mahabharata) “Karna misses target of hitting of fish mark in Draupadi Swayamawar.
5) Mahabharata story has every emotion in book. Sacrifices, Jealousy, Malice, Righteousness, Morality, Love, friendship, revenge, penance, kidnap, cheating.
6) Karna lost war many times in Mahabharata, one in Vana Parva against Ghandharva(when he didn’t able to save Duryodhana and his brothers from Gandharva, after that Arjuan and other Pandava fought with Gandharva though they don’t want to save them, Yudhistira order them to save their cousins), against Arjuna in Virat war, in mahabhara Bhima and Karna able to defeat each other, so does he lost against Styaki(one of Yadav warrior who fought from Padava side and disciple of Arjuna) during war).
7) Kavach and kundal of Karna only can prevent from death from any weapons but not from defeat (please note that defeat and deaths is different).
8) Karna has once defeated Jarasandh, apart from Karna, Krishna, Balrama, Bhima also had defeated Jarasandh.
9) Arjuna and Krishna were Nara – Narayna avtar.
10) After Lord krishan’s death and performing last rituals, Arjuna was going back to Hashtinapur with widowed ladies of Yadavs, some of Robbers attacked and tried to kidnap ladies, Arjuna tried to fight with those Robbers but his arrows from his famous bow ‘Gandiva’ doesn’t seem to be that effective at that time and he wasn’t able to pick his bow ‘Gandiva’ very easily. Story behind is that ‘Gandiva’ bow is given to him for protecting ‘Dharma’ and for specific time only (nearly 64-65 year) by Varuna(previously that bow was held my god including Indra, Brahma). As arjuna never lost any battle in any war and now he lost battle to only few Robbers, he discuss that incident with Vyasa. Vyasa told him that your work to protect religious and your role is over now. So after that all pandava including druapdi left for Swargaarohan. Where everyone died during journey except Yudhister who reaches Swarg in his mortal body.
11) People having many misunderstanding due to Start plus Mahabharata (almost everything wrong), One of them is Drona denied war art training to Karna for being to lower caste, well Karna has taken military training udner Dronacharya, Kruparcharya, Parsurama. Drona only denied him for knowledge of ‘Brahmastra’(he also denied same to his son ‘Ashwathama’, but on his son’s request he teach him ‘Brahmashira’ most destructive Brhma weapon, but he teach him only how to fire that and didn’t teach how to take it revert back), so then he learn some advance weaponry training from Parsurama disguised as ‘Brahmana’. Karna himself declare that during Mahabharat war while telling Shalya about his war training.
12) In some of books it’s written that Bhima was sort of bully against of Dhritastra in post war years, he used to always remind Dhritastra how he killed Duryodhan and his brothers.