千里之堤，溃于蚁穴
qiān​lǐ​zhī​dī​kuì​yú​yǐ​xué

a small leak will sink a great ship

千里之堤，溃于蚁穴 千里之堤，潰於蟻穴 qiān​lǐ​zhī​dī​kuì​yú​yǐ​xué
a small leak will sink a great ship
literally, a dike of a thousand miles may collapse because of an ant hole; small problems or moments of negligence can lead to huge disasters or major losses if not addressed early