A scaphoid is a small bone in the wrist, about the size of an unshelled peanut. It is one of the worst designed pieces the evolution has blessed us with as when broken it doesn't heal well.
The reason for that is that only one small blood vessel enters it and if the scaphoid is broken that blood vessel may not do its job of supplying both pieces of bone with blood, which is required for a bone to heal as it brings calcium and other nutrients from which to restore the bone tissue. It is also fully inside the joint and washed with joint liquid and thus the broken pieces don't stick together well.
It is usually broken when falling on an outstretched hand. Thus learning to roll like they teach in martial arts instead of ending a fall quickly could help in avoiding such an injury.
Many cases initially go unnoticed as they get mistaken for a simple sprain. Tenderness in the anatomic snuffbox is one of the signs of a broken scaphoid as even X-rays don't always show it.
For non-displaced fractures a cast or splint that immobilizes the wrist is usually used. Scientists differ on whether the thumb should be immobilized as well.
For displaced fractures or non-unions a surgical technique called the Herbert screw that keeps the pieces of the scaphoid together is used.
The cast is usually kept on for 6 weeks, while full healing can last up to 12 months even without significant complications.
With complications (which are not rare) or if not diagnosed quickly it can lead to a permanent non-union leading to chronic arthritis and carpal tunnel syndrome.
Thus the point of the story is - if you fall on your outstretched hand and presumably only sprain your wrist, see your doctor. You don't want to find out you still have a broken scaphoid a few months afterwards.
P.S. I broke my scaphoid while jumping over a tennis net backwards 3 months ago. So far, it's hopefully been healing fine, though the cast got uncomfortable at one point and had to be changed.
P.P.S. I am not a doctor so this is is not medical advice. I just happened to read too much about it while worrying about how my scaphoid will heal and thought I'd share.