1) When assassins attacked his home and the fairy killed them all, he thinks that they all left by themselves after she asked them politely because she said "they all left". Even though right before that she told him there are assassins and no one will miss them when they are gone and locked him in the room while she was killing them. For real? Mentality of 5 year old from 94 year old.
2) When he increases his mana to ridiculous amount, and people are telling him "wtf, your amount of mana is 10 times normal magician should have", he still thinks that he does not have that much mana. He also thinks that he is average in strength even though he is OP and that everyone around him should be the same. If he just looks at people around him, he can realize what kind of freak he is, and yet he does not.
3) His sword. Not only he trades a higher level artifact sword for the lower level artifact katana, he does not even freaking suspect anything. When his katana starts to perform above normal sword, he does not investigate it at all and just chills out still thinking it is a normal sword.
Also the part with his sister is so stupid... You can guess what happens just from the already translated chapters.