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'.
Trying to fix a small mistake and ruining the whole project is exactly a case of 'straightening the horns and killing the bull'.
The full proverbial form of the same idea; 矯角殺牛 is the compact yojijukugo version.
From the classical Chinese story in which someone tries to straighten a bull's horns and ends up killing it. The exact historical origin is uncertain, but the phrase is used as a yojijukugo in Japanese.