Fly larvae and simple evidence.

In the Monster of Florence, the investigators ignored simple evidence that cannot be faked and instead relied on the testimony of people with vested interests in the outcome. Instead of taking the evidence of fly larvae on the corpses (fly larvae had no interest in the outcome of the murder investigation), they instead trusted the testimony of people since that testimony fit their theory as to the time of death better than the physical evidence of the body's decay.

When investigating a root cause and gathering evidence to support a solution, keep in mind the types of evidence you are gathering. Often we want a certain outcome to be true and interpret information to fit that outcome. This will often lead you down the wrong path. Try to gather evidence that is independent of the solution you seek. I call this simple evidence. It cannot be faked and can only be interpreted in one way.

If your evidence requires assumptions then it is not simple evidence. Those assumptions may be wrong. Often our assumptions are biased by our experiences our outlook. Try to avoid them.