Reverse causation

The pair. Places with more hospitals have more illness. A careless reading says the hospitals are making people ill.

The other direction. Illness is why the hospitals were built, and why the beds are full. The thing you called the result is the thing that brought the hospitals.

The habit. When two things rise together, check whether the one you called the cause is actually answering the one you called the effect. The arrow may run the other way.