For the majority of Death Note, Light and L are constantly trying to one-up each other to force one another to expose their identity, since, as Ryuk put it, the first one to learn the other's identity will win, while the other will die. While Light manages to keep his identity as Kira hidden fairly well, L is always suspicious of Light, even if he couldn't prove his suspicions before his death.
Light's actions around keeping his identity a secret while still killing criminals under the guise of Kira were fairly well thought out, but there were several times that, even though Light put a great deal of thought into his actions, could have been executed better and may have lead to him escaping justice in the end.
10 Killing Lind L. Taylor Without Vetting Him
Light's first big mistake was taking L's initial test at his word, wasting no time in killing Lind L. Taylor as he masqueraded as L. Had Light vetted the man even a little, rather than simply writing his name in the Death Note, L might not have known for sure that Kira was in the Shinjuku area of Japan, as Light would have figured out that Lind L.
Taylor was actually a criminal being used by L. Even though Lind L. Taylor was arrested in secret, Light has been shown capable of his own investigations throughout the series and would have been able to find at least something on the man.
9 Not Taking Ryuk's Eye Deal
Early in the series, Ryuk offers Light a deal in exchange for the eyes of a shinigami. For the low price of half of his remaining lifespan, Light would be able to kill more efficiently by seeing the true name of anyone whose face he could see. While Misa was wise enough to take this deal, Light was so caught up in his idea of ruling the world as a god for as long as possible that he didn't see the big picture.
Had Light taken the deal, he would have been able to kill L much faster and before any real suspicions came down on him, which also means he may have been able to live longer than he did once Near came into the picture.
8 Killing Ray Penber Instead Of Using Him More
One of Light's more ingenious moves was using Ray Penber to learn the names of every FBI agent in Japan at the time, forcing him to write the names of all of his associates into the pages of the Death Note before killing him. However, Ray might have been more useful to Kira alive rather than dead, as his FBI connections would have been invaluable in learning L's identity.
Rather than simply killing off Ray, Light could have forced him into looking into L instead, which would have also prevented Ray's fiance, Naomi, from catching onto Light so quickly, as she proved to be one of the more capable investigators in the series.
7 Killing Naomi So Soon After Meeting Her
After the death of her fiance, Naomi was quick to realize that Kira was most likely someone Ray was tailing recently and immediately suspected Light since he was the last person Ray was following and was also present for the bus-jacking accident.
After realizing she would be a problem, Light wasted no time in making her commit suicide, though, like Ray, it would have been in his best interest to use her instead, as killing her so soon after meeting her was very suspicious and her suicide didn't go unnoticed by L.
6 Changing His Killing Timelines To Make A Point
After it is discovered that the killings were taking place after school hours, as, unlike the Netflix film's version of Light, he wasn't stupid enough to use the Death Note during school hours, Light purposefully changed the timeline so that criminals were dying whenever he wanted.
Had he done this in the first place, L wouldn't have immediately suspected that Kira was a student, nor would his suspicions have been confirmed that someone with inside knowledge was Kira right after Light changed his timelines.
5 Blatantly Experimenting With The Death Note
Before planning to kill Ray Penber, Light performed many blatant experiments with the Death Note to see how far he could push the boundaries of its rules. While he was able to discover quite a lot about the Death Note, this experimentation didn't go unnoticed by L and lead to his correct assumption that Kira hadn't been killing for very long.
Light also made a point of leaving personal notes for L in the deaths of these criminals, something which would come back to bite him later once L announced himself to Light in person.
4 Admitting That He Was Kira After Losing His Memories
It's hard to fault Light for this mistake since he had no memories of being Kira, but it was a huge error in judgment to admit to L and the Kira investigation that there was a good chance he was, at one point, Kira.
Even if he had no memories of the Death Note, he was admitting to being a mass murderer and potentially subjecting himself to execution unnecessarily, as once the investigation was over, he should have been arrested and tried as a murderer regardless of whether another Kira was still at large in the eyes of the Kira investigation.
3 Killing Higuchi As Soon As His Memories Return
After Light touches the Death Note once again, all of his memories of his life as Kira and the Death Note itself come rushing back. Under the guise of checking each name in the notebook against known victims, he kills Higuchi without hesitation using a hidden piece of the note hidden inside his watch.
Although the Kira investigation was also dealing with the revelation of Rem's existence at the time, it was suspicious enough for L to rethink his dismissal of Light's current state as Kira, which forced Light to kill both Watari and L and put him on Near's radar.
2 Joining The Kira Investigation To Investigate L
While joining the Kira investigation under the guise of helping them find the real Kira, working so close to L only served to put him in the spotlight even more, as L wasn't as suspicious of Light while they weren't working together.
Had Light remained in the shadows, he could have been able to feign ignorance rather than having to resort to killing so many investigators, though he would also have had to deal with Misa's existence from a distance as well.
1 Putting His Trust In Mikami
After Near becomes rightfully suspicious of Light, he decides to pass the role of Kira onto another man, Mikami, for a while to shake off Near's suspicions.
While Mikami had Light's view of justice and was a good choice on paper, he was also clearly unstable, even more so than Light himself, and this instability is what would eventually lead to the pair dying by Near's hands in the final episodes of the series.