I think it is well known that if we indulgently let something bad happen, there will be more similar cases arising. The outbreak of the negative effects could be unstoppable. So is there any idiom, proverb or fancy expression used to stress this point.
A stitch in time saves nine.
Don't spoil the ship for a ha'porth of tar.
For want of a nail, the shoe was lost;
For want of the shoe, the horse was lost;
For want of the horse, the rider was lost;
For want of the rider, the battle was lost;
For want of the battle, the kingdom was lost;
And all from the want of a horseshoe nail.
There is also an expression
the thin end of the wedge
for which Merriam-Webster has
the beginning of something that will become more serious, unpleasant, etc.
The bank's decision to raise rates could be the thin end of the wedge if other banks follow along.