Beta-Carotene
Beta-carotene (B-Carotene) as a dietary supplement is proved by science (through randomized trials, not observational) to INCREASE the risk of dying. So it does the opposite of it aims to do on the label. In other words, those who take Beta-carotene, die 7% more based on the scientific randomized study below. Thus Beta-Carotene use is actively discouraged by scientists behind these studies.
Here's a more detailed video on that study discussion from University of California San Francisco Department of Medicine: