trying to fix a small flaw and ruining the whole thing
A yojijukugo (four-character idiom) used to warn against overcorrection that causes greater harm. Literally 'straightening the horns of a bull and killing it in the process'. Equivalent to 'the cure is worse than the disease'.
小さなミスを直そうとして企画全体を台無しにするのは、まさに矯角殺牛だ。
Trying to fix a small mistake and ruining the whole project is exactly a case of 'straightening the horns and killing the bull'.