Even those who are not fans of the Harry Potter series are likely to know the name Voldemort. He is the most famous dark wizard in the franchise and the original villain that fans of the Wizarding World got to know. However, despite his fearsome renown, he was still foiled by a barely qualified Harry Potter.
It seemed improbable that a boy that hadn't even graduated school would have the power to destroy the most powerful dark wizard of all time, but he still managed it. This was due less to his skill than the errors that Voldemort himself made. Between his underestimation of the wizards around him and his complete lack of understanding of deeper forms of magic, Voldemort's mistakes are what resulted in his defeat.
Making Horcruxes
Voldemort's biggest fear was death, and he would have done anything to keep his inevitable end from happening. While he was still in school, he began research in the dark arts to find a solution to mortality, and he found Horcruxes. He successfully used murder to split his soul into multiple pieces, hiding those pieces to keep them protected.
However, with his soul so fragmented, he ended up putting himself in a weaker state. This ultimately resulted in his soul unintentionally splitting again when his curse against Harry rebounded. By accidentally placing a part of his soul Harry, he created a wizard that was uniquely equipped to destroy him.
Talking To Slughorn
During his research of Horcruxes, Tom Riddle started to wonder if it was possible to split his soul more than once. With magical significance existing within the number 7, he felt that that number of soul fragments would strengthen him. But young Voldemort needed more information before moving forward with this plan, so he turned to his professor, Horace Slughorn.
Voldemort put his faith in no one, but he anticipated that if the day ever came that Slughorn would be questioned about what he had told him, Slughorn's value in his reputation would prevent him from ever sharing. While he was correct, Voldemort underestimated the power Harry would have in coaxing that information out of Slughorn.
Underestimating House-Elf Magic
Voldemort set up extensive magical protection around the hiding places of his Horcruxes. To properly secure one, he borrowed a House-elf from one of his followers, Regulus Black. That elf was none other than Kreacher, who accompanied Voldemort into the cave to store Slytherin's locket.
In a book scene from Deathly Hallows that would have been impossible to adapt to the movie, Voldemort forced Kreacher to drink the deadly poison to test its efficacy and left him there to die. There was no way for a wizard to disapparate from the cave, but Voldemort wasn't aware that this restriction did not apply to a House-elf. Kreacher returned to Regulus, who then used Kreacher's information to retrieve the locket himself.
Assuming No One Knew About Hogwarts Secrets
Voldemort was so arrogant that he believe that no one else in the history of Hogwarts had ever discovered the deep secrets of the castle. Therefore, when he hid one of his Horcruxes in the Room of Requirement, he believed no one would ever find it.
Of course, hundreds of other students had discovered the secrets of this room, including Harry and Malfoy. Voldemort had been so sure of the diadem's safety that when he realized Harry was after his Horcruxes, he didn't bother going to retrieve it. This error truly shows Voldemort's hubris, and greatly contributed to his defeat.
Giving Lucius The Diary
During Voldemort's time at Hogwarts, he was successful at finding and opening the Chamber of Secrets. However, out of fear that the school would be closed and that he would lose his home, he decided to close it again. He placed a fragment of his soul in a diary, planning to later plant it in the school to ensure that the Chamber would again be opened.
Sometime before his disappearance, Voldemort entrusted this diary to Lucius Malfoy. Lucius was supposed to keep it safe, but with the Dark Lord gone, he used it for his own purposes. Upon his return, Voldemort discovered that the diary had been destroyed and punished Lucius severely, but the damage to his soul had already been done.
Listening To The Prophecy
The only reason Voldemort ever went after Harry to begin with was because he learned of a prophecy that predicted his end at the hands of a boy that would be born at the end of July. He targetted the one that he believed the prophecy spoke of, but his attack only resulted in him giving his powers to the baby, uniquely equipping Harry to carry out the prophecy.
Dumbledore reveals to Harry that if Voldemort had simply ignored the prophecy, it would have never come true, which is a detail from the Harry Potter books everyone forgets. By hearing it and acting on it, he ensured that Harry's mother would sacrifice herself for him, that Harry would become a Horcrux, and ultimately, that Voldemort himself would be defeated.
Trusting Narcissa
After Voldemort failed again to kill Harry but was unknowingly successful at destroying the part of his soul that lived in him, Voldemort ordered Narcissa Malfoy to check if Harry was dead. What he should have realized was that Narcissa would have done anything to get back to her son at the castle, even turn on her master.
Once Harry silently confirmed that Draco was alive, Narcissa falsely told Voldemort that Harry was dead so that they could all return to the castle. Voldemort trusted none of his followers, however, his arrogance once again let him believe that they feared him too much to deceive him.
Assuming The Elder Wand Transferred With Murder
When Harry's wand regurgitated Voldemort's powers back at him, Voldemort knew he needed to find a powerful wand that could defeat Harry's. He started his quest for the Elder Wand, which he ultimately discovered was buried with Dumbledore. Once he had retrieved it, he believed himself unstoppable.
The wand was reluctant to work for him, however. He deduced correctly that the wand was loyal to another, but incorrectly assumed that it was to the person who had killed its previous owner: Snape. What he didn't know was that the wand only needed to be won by force, not murder. In one of the most shocking things to happen in Harry Potter, the wand was loyal to Harry in the end, meaning that it would not kill him.
Trusting Snape
Voldemort was not a trusting man, but due to his nature to value power, he assumed that other wizards were the same. Snape had first come to Voldemort as a loyal follower looking to gain power and glory. However, after losing his childhood love, Snape became devoted to his mission to protect Harry and bring Lily's murderer down, which Snape was dedicated to until his dying moment.
To his end, Voldemort assumed that Snape's interest in Lily was only desire. Voldemort had no understanding of the kind of love that Snape had for Lily and therefore could never have seen a reason for Snape to have turned against him. This trust in Snape's thirst for power made him blind to the ways that Snape was sabotaging Voldemort's goals.
Underestimating The Power Of Love
As the best Wizard in Harry Potter history, Dumbledore knew that the power of love that would ultimately lead to Voldemort's defeat. Voldemort, who had never experienced love in his life, was unable to recognize the very real magical power that came along with love, and therefore he was unable to ever fully fear what love could do.
Lily's sacrifice for her son, Narcissa's deception to get back to Draco, Snape's work as a double agent, Harry sacrificing his life in the forest for his friends, all of these actions contained the mysterious magic of love that Voldemort had never given a thought to. If he had understood how superior this magic was to his own, he would have never underestimated it, and never would have lost.