Learn English – Opposite of ‘damned with faint praise’

Misfortunes never come singly

This proverb (self-explanatory) means that bad things tend to happen in groups. For example:

I already told you that my wife lost her job. Well, misfortunes never come singly; our house was robbed last night.

Less idiomatic, but closer to question: one error leading to another

As per my comment above, consider the fewer, the better fare.

The More The Merrier The larger the number involved, the better the occasion. For example, John's invited all his family to come along, and why not? The more the merrier. This expression was first recorded in 1530, when it was put as “The more the merrier; the fewer, the better fare” (meaning “with fewer there would be more to eat”), an observation that made its way into numerous proverb collections.

The American Heritage® Idioms Dictionary