When people think about a will that is not valid, they often think about a fake will. Maybe, after a parent's passing, an heir drafted a will that gave them a far greater share of the estate than they got in the original. There is a clear chance for financial gain.
This type of thing does happen, but a fake or fraudulent will is not the only type of invalid will. There are many other ways that a will can fail to hold up in court. One such issue is if the person who signed it was pressured into doing so.