Reason 2: They didn’t ask the right questions
A scientific experiment gives you the answer to a specific question (e.g. does this intervention achieve the specified outcome better than a specified control in specified conditions). As mentioned in the previous post, this does not tell you if the intervention works in other conditions. However, there are also many other important questions that this research does not answer — for example, is the intervention acceptable to the community?; Is the outcome desirable?; Is the intervention safe?; Is it cost effective?; etc. Sometimes subsequent research asks a different question and the answer means that the policy decision based on the initial research is reversed. For example, research might demonstrate that treating a certain crop with a certain fertiliser increases yield. However, some years later further research may indicate that the fertiliser is damaging to the local environment and thus the fertiliser is withdrawn. This does not mean that the original research was wrong — the fact that the fertiliser increased yield is still true. But the additional information gained by asking a different question has changed the policy decision.
Next — Reason 3: They lied (or at least stretched the truth!)