Let's say that you are one of the beneficiaries of someone's estate. As the months and years pass by, you always assumed that you were entitled to some portion of the estate because the individual in question told you so. When the day comes and that person passes away, the executors of the will and the estate plan inform you that actually don't have any claim to the estate.
Was the person lying to you this whole time? Or was a mistake made during the construction and execution of the will and/or estate? Depending on the circumstances and the state, you could have the grounds -- and the rights -- to challenge the will or estate and try to obtain what you think was truly meant for you.
For example, sometimes spouses believe certain assets belong to them when their significant other passes away. This is a complex issue, and it can become further complicated in states where community property laws are in effect (Colorado does not observe community property laws).
Children and grandchildren also often believe they are entitled to something or that they have rights in the area of inheritance and estate plan law. However, generally speaking, these people usually do not have a direct right to property from an estate that they would presumably be a part of. There are still situations where these people could be vindicated though, such as when an unintentional or accidental omission of the individual in a will or estate plan occurs.
Source: FindLaw, "Inheritance Law and Your Rights," Accessed Dec. 10, 2014