To think oneself clever
Refers to when one thinks oneself clever and makes decisions based only off of one's own opinions
Negative
笨人的可怕不在其笨，而在其自作聪明 | Bèn rén de kěpà bùzài qí bèn, ér zài qí zìzuòcōngmíng
The scary part about stupid people isn't their stupidity, but rather the fact that they think themselves smart
[Source]
这位编辑自作聪明，把作者原稿的精华部分删改了 | Zhè wèi biānjí zìzuòcōngmíng, bǎ zuòzhě yuángǎo de jīnghuá bùfèn shāngǎi le
The editor thinks himself so clever that he modified the most important part of the author's original draft
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《尚书》(Shàng Shū), generally known in English as the "Book of Historical Documents" or "Book of History." It is one of the five classics of ancient Chinese literature.
This chengyu is from a line in the work listed in the "Origin" section above