Researchers announced recently that over the past 25 years the incidence of skin cancer caused by exposure to harmful rays from the sun has continued to grow in spite of the increasingly widespread use of sunscreens.
Conclusion
Using sunscreen is unlikely to reduce your risk of skin cancer.
Evidence
Over the past 25 years, more and more people are using sunscreen, but the incidence of skin cancer caused by harmful sun rays has continued to go up.
Evaluation
Anti-causal arguments such as these can be weakened by showing an important difference between the two groups / cases being compared.
The group 25 years ago used less sunscreen, but had a lower incidence of skin cancer than the group nowadays. But maybe something else is different about them that preserves our ability to think that sunscreen is actually effective.
Maybe the sun's rays are more harmful now (bigger holes in the ozone layer?), so we'd expect 50% more skin cancer, but because of sunscreen's effectiveness it's only 20% more skin cancer.
Maybe people are spending way more time in the sun, so we'd expect 50% more skin cancer, but sunscreen is keeping it down to only 20% more.
Maybe people don't put on sunscreen as frequently / effectively as they used to.
Goal
In order for us to argue that sunscreen does reduce your risk of skin cancer, we need a way to make an excuse for the fact that the incidence of skin cancer is rising. So we need something that makes today's sunscreen wearers more likely to be getting skin cancer than their counterparts from 25 years ago.