There are a couple of things to initially know about wills. The first is that they are absolutely vital documents that any adult needs to have. It doesn't matter if you are a young adult and you may not have the assets that someone older than you may have -- it is still important to have a will. The other thing to note about wills is that they are usually uncontested, and even if they are, most of the time the will remains intact and is administered as the deceased individual wanted.
With that said, there are certainly circumstances where a family member, loved one, or heir may contest a will successfully.
One of the intricate circumstances that can lead to a will being challenged is called testamentary capacity. What this essentially means is that the person who is challenging the will is saying that the testator (the individual who made the will) did not comprehend the consequences of his or her provisions in the will when they made them.
There are some other simpler circumstances that lead to someone challenging a will. For example, if the will was fraudulent, then someone could challenge it's execution. The challenger could also argue "undue influence" if another person applied pressure on the testator to include certain provisions or language in the will.
Another way that a will could be challenged is if another will exists. If that second will is legitimate, it would invalidate and override the first will that existed, and that could dramatically change the way that the individual's estate is administered.
Source: FindLaw, "Reasons to Challenge a Will," Accessed June 19, 2015