There is a strong coorelation between drinking coffee and surviving cancer.

But it turns out the reason for the coorelation is probably because people that suffer from cancer wasting lose their appetite and their desire to drink coffee. So the cancer causes the loss in coffee drinking, rather than the coffee drinking causing a survival of cancer.

Unfortunately epidemiological studies seem to show a lot of bullshit... and some things appear to have a very strong effect... but very probably don't have any effect at all (given other data I will not clutter this post with).