When we apply sunscreen, we tend to go part by part. First, we'll do the shoulders, then the upper arms, then the lower arms. Unfortunately, some parts, like the crook of the elbow, get left out because they're in a sort of no-man's-land.
That's probably why so many people forget to lather up the back of the knee. You may apply sunscreen to your lower leg, knee and thigh, but the back of the knee routinely gets snubbed. This is especially apt to happen if you apply sunscreen to your legs while you're sitting -- if your leg is bent, you may not realize you haven't covered that crease. But once you flip over to sun your back, the skin is exposed to the sun. A burn behind the knee makes it uncomfortable to walk, so give this part of the leg the attention it deserves.