I have a very sensitive skin. Unprotected in full summer sun (latitude 51 north), I'll burn within 15 minutes. In typical hiking attire, only my hands are in the sun, and I keep them in my pockets a lot of the time. My cheeks and lips receive only indirect sunlight, the rest is covered by a hat, sunglasses and a beard. In this attire, I've had sunburn through t-shirts a few times. They are never too bad, though, as I'll overheat way before I burn.

In non-fully protected clothing, something else burns before my chest has the chance to. A major problem with sunscreen is that it degrades over time, and is severely affected by sweat and abrasion. I've read that you should be reapplying every 30 minutes. I typically don't reapply nearly that often, so some parts of my skin are not fully protected. Once those start to burn, I leave direct sunlight, so the skin under my t-shirt (which has consistent coverage) never really gets the chance to burn.