If someone you know has left a will that you don't agree with, you may have legal grounds to contest that will. Not being left a certain inheritance isn't always enough to have the will set aside and an estate go through probate court, but there are times when it's actually encouraged for a person to contest a will. Note a few common reasons for this so you know if you should contact an attorney and at least have the will reviewed.
Reduced mental capacity
For a will to be legal and valid, the person making the will needs to have understood what they were doing. If the person was suffering from dementia, was having a medical emergency or if there are any other reasons to assume they were not in their right mind, it can be good to contest it and have it evaluated by a judge.
Do you have any questions about the signature of the person making the will or the persons who witnessed it? If anything seems questionable, including the circumstances surrounding when the will was supposedly signed, you might ask an attorney how it can be contested and reviewed, just to ensure the execution and signature were both valid and accurate.
Note, too, that there are often laws about who can witness a will. If the witnesses were underage at the time, or you have any reason to believe that their signatures may have been forged or falsified, this can also give you reason to contest the will.
Lack of knowledge
A person who signs a will has the legal right to know all the details of the will, including the implication of leaving an inheritance to others. If that person did not know how the will was worded or may not have understood terms of the will, this can be reason to contest it.
This is different than diminished mental capacity; a person may have full mental capacity but may have been misled about the contents of a will, the legalities of how a will should be prepared and other such issues. Don't assume that only someone with diminished mental capacity would have a lack of knowledge, but talk to an attorney about how this legality may apply in your case.
If a person was threatened, coerced, blackmailed or otherwise influenced to prepare and sign their will, you may have legal reason to set it aside. This type of influence can include any type of threat, such as the threat to be put out of their home, or even religious influence, such as telling a person they will 'burn in hell' or suffer some other type of consequence if they don't adjust their will.