You can probably imagine this scene in a movie: a grieving family is together in a room with a lawyer as they prepare to learn what their deceased loved one has left them in their will. The lawyer looks over the documents and start reading the deceased person's last wishes, and as he goes down the list he finally arrives at the treasured home that everyone wants to receive. The lawyer reads off the name of the beneficiary, and everyone gasps in surprise: it's the immature family member that did nothing to "deserve" the asset.
This problem is not reserved to a movie screen. It is not something that only occurs as a plot point in a story. It is a real issue that can happen at any moment. People can feel jilted about the inheritance that they receive.
But who can challenge a will, and why are they able to do so? To answer this question, we need to know what "standing" is. Standing varies from state to state depending on the laws, but in general this means that a person has a legitimate interest in another person's estate. If they are named in a will; if they would have been in the will if undue influence over the grantor didn't occur; if they stand to gain from an invalid will; all of these circumstances can give someone "standing."
Having "standing allows someone to legally challenge a will, whether they are an heir, a beneficiary or a minor. Obviously the specifics of each case will dictate how the challenge proceeds, so you should consult with an attorney if you are considering mounting such a legal challenge.