One rotten apple in the basket infects the whole.
More Dutch Proverbs:
The best horse stumbles sometimes.Dutch
Don't yoke the plough before the horses.
Dutch
Escaping from the smoke he falls into the fire.
Dutch
Spare at the spigot, and let out the bunghole.
Dutch
It sticks to his fingers, like the charity-money to the matron.
Dutch
Strain not your bow beyond its bent, lest it break.
Dutch