千里之堤，毁于蚁穴
qiān​lǐ​zhī​dī​huǐ​yú​yǐ​xué

a slight oversight can lead to disaster

千里之堤，毁于蚁穴 千里之堤，毀於蟻穴 qiān​lǐ​zhī​dī​huǐ​yú​yǐ​xué
a slight oversight can lead to disaster
literally meaning a thousand-mile dike can be destroyed by an ant hole; used to describe how a tiny mistake or a small amount of negligence can cause a huge catastrophe