Reason 3: They lied (or at least stretched the truth!)
Let’s be completely honest. Scientists are human beings and human beings lie. There are numerous cases where the results that scientists report are in fact fabricated. Some of these cases are high profile — such as the fraud committed by Korean scientist Hwang Woo-Suk. In addition to blatant fraud, there are also plenty of cases where scientists unconsciously inflate effects because they believe that they exist. There is a detailed (and sometimes quite amusing) record of scientific results which have been retracted for various reasons online.
Various strategies exist to counteract human error (or fraud) in scientific findings. For example, the concept of double blinding in clinical trials, where neither the patient nor the scientist knows which patients have received the ‘true’ drug, aims to prevent scientists unconsciously over-reporting effects in the treatment group. The principal of reproducibility also counteracts false findings. If a scientist has faked his data, it is likely that other scientists will not be able to reproduce the findings and therefore the idea will not be considered scientifically proven.
See Reason 4: The scientist misinterpreted the results tomorrow!