In the Death Note series, L was a genius that solved every case he was involved in before encountering Kira, a mass murderer capable of killing through supernatural means. Despite being the main opponent of the series protagonist, Light Yagami, the character still counts as a fan favorite. Famous for his awkward sitting posture and many other unnatural mannerisms, L possesses astounding intellect and unbelievable tactical prowess.
Unfortunately for the great detective, however, he is bested in the individual battle against Kira, ultimately costing him his life. Throughout the battle, L's assumptions were mostly accurate, but the absence of solid proof greatly hindered his ability to take action. So, despite possessing seemingly flawless skills, the eccentric crime-fighter wasn't impervious to making occasional, critical blunders.
8 Working Closely With The Japanese Police
In order to catch Kira, L enlists the help of multiple organizations. One of them is the NPA, within which he forms the Japanese Task Force – the sole organization in the country saddled with the responsibility of catching Kira consisting of multiple police officers.
However, the group's competence came into question when L realized that someone within the task force was leaking confidential information, inadvertently allowing Kira to alter his actions based on the direction the investigation was going. As a result, leaving important records of an anonymous criminal in the hands of the local police led to the loss of an extremely valuable lead.
7 Revealing Himself To Yagami Light
In the 9th episode of the series, L confronted Light directly, aiming to cause a slip-up where Light would end up exposing his identity as Kira. It was not an easy choice, but it was one he deemed necessary as all efforts to catch the mass murderer made up to this point had been fruitless.
However, the flaw in this approach lies in the fact that Light is not easily flustered. He is a frighteningly good actor capable of hiding massive shocks behind an impenetrable mask. Light managed to suppress his natural reactions and casually interact with L as a regular university student throughout their time together, ultimately rendering L's decision futile with the cost of revealing his identity.
6 Paying Attention To Kira's Meaningless Clues
In an attempt to narrow down suspects related to members of the task force, L requested assistance from the FBI, who in turn granted him 12 agents to aid his cause. However, with each agent tailing different suspects while following a specific schedule, L decided it was okay to focus his attention elsewhere; the prisons.
Prison inmates that had fallen victim to Kira started to display strange behavior right before their deaths, with some leaving behind symbols and notes believed to be the words of the mysterious killer. On further inspection, L discovered that these notes contained cryptic messages and chose to devote himself fully to uncovering the meaning behind them. Unfortunately for him, they were meaningless, random words communicated by Kira to keep the detective occupied while he murdered all 12 FBI agents sent to Japan.
5 Conducting the Gun Scare
After deciding to let Light and Misa out of their holding cells following over 50 days of confinement, L convinced Souichiro conduct a final test to ascertain if either of them truly possessed the power of Kira. It was staged right after their release, with Souichiro willing to risk his life if L's suspicion proved true.
Though he had no idea of knowing at the time, this ruse was pointless from the beginning, as he later finds out that Kira's ability to kill comes from a notebook. While he would always be insistent on suspecting Light and Misa, this attempt only served to ease the suspicion of those around him, making it harder for him to take action in the future.
4 Releasing Amane Misa From Confinement
Unlike Light, there were solid grounds for Misa's detainment. The task force possessed hard evidence of her involvement as the Second Kira and could consider her relationship with Light as an added bonus. The decision to release her was unnecessary even though she had lost her memories.
If L had chosen to keep Misa in detention regardless of the fact that it would earn him the ire of the remaining task force members, Rem would eventually take care of Light. Though this method may seem lacking in moral integrity, it would have served as an efficient means of dealing with both culprits.
3 Agreeing to Detain Yagami Light
In the 16th episode, Light requests to be taken into custody and placed under surveillance. Light explained to the group that he feared he may be Kira and suspected the killings were performed while he is unconscious. After hearing this, L is initially skeptical but eventually agrees, not willing to miss an opportunity to observe Light in such favorable conditions.
Considering all the events which had transpired up until now, it is fair to say L had no reason to agree to Light's request, as he had already been placed under surveillance once before and observed even outside his home. Furthermore, any efforts that involved observation proved fruitless as Light has repeatedly displayed that he can kill under any given situation. Yet, L chose to accept his request, a decision that ultimately led to further confusion.
2 Overlooking Misora Naomi
Misora Naomi was a former FBI agent who made her first appearance in the 5th episode of the anime. She was also the fiancée of the first FBI agent killed by Light. At a point in the story, her existence was considered the missing piece in L's puzzle and the crucial oversight in Light's planning, highlighting just how important she was at the time. Following the death of her fiancé Raye Penber, she wasted no time before deciding to take action, determined to avenge her lover and bring Kira to justice.
Despite being retired, she possessed impressive intuition and vaguely deduced the identity of Kira based on a crucial moment during her fiancé's investigation. Unfortunately for L, she arrived at the task force headquarters to present the evidence but was received by Light, who cleverly dealt with her before she became a problem. This could have been avoided though; a fact L indirectly acknowledges himself when he requests for one person to be stationed at the task force headquarters at all times.
1 Allowing Light Reclaim The Death Note
After relinquishing ownership of the death note and suffering from selective amnesia, Light regained his memories in the 24th episode of the anime. It is an extremely crucial moment and one of Light's most elaborate plans in the series, spanning over a period of 3 months. However, the success of this plan could have been halted if L had kept the notebook away from Light.
Light was still under mild suspicion from L despite losing his memory for over 3 months, realistically indicating how sure L was that Light had somehow been involved with the notebook at one point in time. L fails to act on this suspicion, following the arrest of Higuchi. Upon receiving the note, L's major priorities should have involved keeping it away from Light, or at least keeping a close eye on it after it was snatched away, which would have revealed Light writing Higiuchi's name in blood as well.