Штопай дыру, пока невелика

If you take instant actions when something starts going wrong, you will not have to face bad consequences. See Упустишь огонь - не потушишь (У)

Cf: A dropped stitch is soon a hole (Am.). A green wound is soon healed (Am., Br.). Не that corrects not small faults will not control great ones (Br.). Не that repairs not a part, builds all (Br.). A little neglect may breed great mischief (Am.). Nip the act while in the bud (Am.). A step in time saves nine (Am.). A stitch in time saves nine (Am., Br.). Who repairs not his gutters repairs the whole house (Br.)