1. Sample bias: "Why just study WEIRD [1] subjects? Let's do male Harvard graduates!" (Yes, half the study included inner city men, and one eighth of the duration featured women. It's still super heavily biased towards Harvard men.)
2. Correlation is not causation: "Hmm, health is correlated with relationship satisfaction. Could there be a common cause for both? Or maybe people like to hang out with healthier peers? No, the clear conclusion is that working on your relationships magically makes you healthier."
3. Inconsistent data collection: "Those '30s nincompoops were measuring skulls and handwriting. We'll stop doing that and take MRIs instead. But it's still the same study!"