Common errors in science

I've found myself falling for health advice or arguments "backed by science" that have bitten me. I'm collecting a list of fallacies and errors (ie in the use of statistics) so I can better spot them myself.

• Medical studies often do before-and-after comparisons, either with or without a control. However, it turns out that even comparisons with a control can produce misleading results when compared to randomised data. Source, Example