Mahabharata is such a vast treasure house of facts and knowledge that it is impossible for anyone to know everything! Many of the facts are missed out or unknown. These are some of the facts that I collected from various sites and through my own research. (Although there may be some facts that have collected from folktales and such.)
1. Sahadeva knew everything about the future. He knew that a war was going to occur but he kept silent because he was under a curse that if he revealed anything to anybody he would die.
2. Krishna had a sister named Ekanaga born from Nanda and Yashoda. (Known as Ekanamsa in Harivansha)
3. Bhishma knew that Shikhandi was the woman Amba from his past. This was why he lowered his arrows whenever Arjuna used her as a shield.
4. Bhanumati (Duryodhana's wife) was a devotee of Lord Krishna.
5. Balarama was Abhimanyu's father-in-law. His daughter Vatsala was Abhimanyu's wife. Actually Balarama wanted to marry Vatsala to Laxman (Duryodhana's son) but Abhimanyu did not like this because he loved Vatsala. He asked his brother Ghatotkacha to intervene. Ghatotkacha went to Laxman's marriage ceremony and frightened the groom. He brought Vatsala for Abhimanyu by magic. Laxman, traumatized from the fright vowed never to marry. Duryodhana's anger was further fueled when he learned about Vatsala's marriage to Abhimanyu.
6. Lakshmana the sister of Laxman was married to Samba(Krishna's son).
7. Balarama had originally planned to marry Subhadra to Duryodhana. But Subhadra wanted to marry Arjuna so she ran away with him.
8. Arjuna's 'pilgrimage' for breaking his marital laws with Draupadi earned him three more wives. They were: Chitrangada (Manipura), Ulupi (Naga) and Subhadra.
9. Abhimanyu was actually the soul of a daitya named Kalyavana. Krishna after burning Kalyavana to death had captured his soul (tied him in a knot of his upper cloth) and taken him to Dwaraka only to stuff him into a cupboard. Ghatotkacha(who had been sent by Hidimba to take care of Subhadra in Dwarka as Pandavas were in the forest.) saw Krishna and got a scolding from him. He went and reported the matter to Subhadra who went downstairs to check. She opened the cupboard and a light entered her womb which caused her to fall unconscious. This was the main reason why Krishna divulged only half the secret of Chakravyuha to Abhimanyu when he was in the womb.
10. Ekalavya was actually Krishna's cousin. He was the son of Devashrava (brother of Vasudeva) who got lost in the forest and was found by a Nishada Hiranyadhanu. Ekalavya died protecting his father during Rukmini Swayamvar. He was killed by Krishna.
11. Yudhistira had another wife named Devika. She was the daughter of Govasana of the Saivya tribe. Her son was Yaudheya.
12. Shikhandi had a son named Kshatradeva. Shikhandi was married to a Dasrana princess.
13. Karna lost his first son Sudaman(16 year old) in Draupadi Swayamvar. He also lost his foster brother Shatruntapa(Shon) in Virat war. Both were killed cruelly by Arjuna.
14. Krishna took charge of Vrishaketu, Karna's youngest son after Mahabharata war. Krishna taught him all the divya astras but told him not to teach them to anybody else as the people of Kaliyuga would take advantage of it.
15. Lakshmana went sati on Samba's pyre.
16. Arjuna was unable to defend the wives of Krishna from common robbers after Krishna's departure to Vaikuntha. His bow became heavy and he forgot all his mantras. The 8 main wives committed suicide. The others were kidnapped by robbers.
17. The 16,000 wives of Krishna were actually incarnations of Apsaras.
18. Yuyutsu was the half-brother of Duryodhana. He was the son of Dhritrashtra's Vaishya wife. Duryodhana had appointed him as the finance minister. Later on he was appointed as Parikshit's caretaker and the regent of Hastinapura. He looked after the throne until Parikshit was old enough to rule. Yuyutsu committed suicide when he couldn't save Parikshit from Takshaka.
19. Dhritarashtra and Gandhari retired to the forest 15 years after Yudhisthira's coronation. The main reason for this was Bhima's taunts which depressed them.
20. Parikshit's wife Madravati was a frog!
21. Draupadi laughed at Bhishma in his last moments while he was giving them knowledge he possessed.
22. Duryodhana had gone to Dwaita forest to humiliate the Pandavas with all the servants from Indraprastha. There the Gandharava Chitrasena and his apsaras were enjoying in the pond. Duryodhana asked Chitrasena to abandon the pool but as he refused, a heated argument broke out between them. Chitrasena attacked Duryodhana. The Pandavas who were nearby asked Chitrasena to let him alone. Chitrasena who was a friend of Arjuna spared Duryodhana but not before tying him with vines and setting him on his horse. Duryodhana though humiliated, thanked Arjuna and asked him what he wanted for the turn he had done to him. Arjuna said that he would ask from him some other time.
23. During the war, Duryodhana accused Bhishma of not using his full power because he favoured the Pandavas. Bhishma in his anger took 5 gold arrows and infused them with mantras. He vowed that the Pandavas would die before sunset the next day through these arrows. Duryodhana suspected that Bhishma would never use those arrows so he kept them with him and decided to give them to Bhishma the next day on the brink of the war. Krishna on the other hand reminded Arjuna of the boon Duryodhana had promised him. He told Arjuna to ask for 5 gold arrows from Duryodhana. Arjuna did so. Duryodhana was surprised but did not grudge Arjuna and gave the arrows away. Duryodhana asked Arjuna. "Who told you that I have such arrows?" Arjuna replied. "Krishna, who else?"
24. Karna was the first person in attempting to kill Jarasandha. He did this in Bhanumati Swayamvar when Duryodhana ran away with the princess. He grabbed Jarasandha's knees in wrestling and started pulling them apart. Jarasandha immediately apologized and made Karna his friend.
25. Draupadi was asked 3 times by Duryodhana to come to the court after her husbands lost her in dicing. Draupadi kept denying and asked Duryodhana to question Yudhistira whether he staked himself first or her first. Duryodhana asked Draupadi to question her husband in court directly. Yet Draupadi refused to come and sent the attendant back. Duryodhana lost his temper and ordered Dusshasana to bring her there at any cost.
26. The disrobing of Draupadi was instigated by Karna not Duryodhana. Duryodhana has been remarkably quiet during the entire episode except while asking Draupadi to come in court.
27. The Akshaya Patra was obtained by Yudhistira from the Sun God for Draupadi and the Brahmanas.
28. Arjuna skipped his 5 years of exile by visiting Indra for the Divya Astras. During this time the others performed a pilgrimage with the Brahmins and had their first encounter with Babreeka (Bhima's grandson).
29. Balarama's wife Revati was from the 1st Manavantara. She was so tall that Balarama used his plough to pull her down to the right size.
30. The Upapandavas (Pandavas's sons) all died unmarried.
31. Duryodhana expressed deep regret when Ashwatthama killed the Upapandavas in a fit of rage in their sleep on the 18th day.
32. Flowers rained on Duryodhana when he was felled on the 18th day by Bhima.
33. Bhima had one surviving son named Sarvaga by Balandhara who was not given the throne despite being much elder to Parikshit. He became the ruler of Kashi(his mother's land) instead.
34. Bhanumati and Balandhara were cousins who were married to the two fierce rivals in Mahabharata.
35. Drupada had 10 more sons in addition to Dhristadyumna.
36. Satyaki's real name was Yuyudhana.
37. Draupadi asked for Ashwatthama's Chintamani when Arjuna and Bhima catch him on the last day of the war. She says that she shall not eat or sleep until her wishes are fulfilled. The mani is then fixed onto Yudhistira's crown.
38. Yudhistira learnt to play dice from the Sage Vrihadaswa who narrated the story of Nala and Damayanti in their exile.
39. Raja Janaka the ruler of Mithila(Videha) was still alive when Balarama visited him while chasing Satadhwana for the Syamantaka jewel.
40. Kichaka was 30 years old when he proposed Draupadi. Draupadi herself was 50!
41. Duryodhana was born on Ashwin Krishna Dashmi. His Nakshatra was Bharani.
42. Yudhistira was born on Kartik Shukla Panchami. He was two years elder to Duryodhana. His Nakshatra was Jyeshtha.
43. Akrura (Kansa's charioteer) enjoyed the power of the Syamantaka Mani for 52 years before Krishna caught him out. Still, he was allowed to keep the jewel as it was proving to be beneficial to all.