What are examples of the fallacy of causation?

There are two different fallacies you might be thinking of:

The fallacy of the single cause is the assumption that one thing alone led to another, when multiple factors applied. This is often seen after tragedies, where the urge to simplify helps people deal with the situation. For example, during Hurricane Katrina much of the disaster was blamed on the failure of the levees and flood walls, but much of the loss of life was also due to poor evacuation and rescue efforts (and even that is a vast simplification).

The fallacy of correlation and causation is the assumption that because one thing happened after another, it was caused by it. For example, some years ago researchers noticed that women taking hormone replacement therapy (HRT) had a lower-than-average incidence of coronary Heart disease (CHD), leading doctors to propose that HRT was protective against CHD. But randomized controlled trials showed that HRT caused a small but statistically significant increase in risk of CHD.