Hot button issues, like toxicity, health care, education, evolution, weight loss, endangered species, sleep, habitat, oral hygiene, etc. Just about anything related to either people, land, products, processes, faith, or stuff has someone who would like to influence the results.

Even if it's simply do the experiment 100 times until you get the result you want. Or just quietly pull the plug when it's not going the way you hope. There are many ways to manipulate even honest research done in good faith.