First of all, did the decedent have a valid Will? Will Caveats are a category of claims that have to do with determining if a Will is valid, or which Will is valid, or if there is no valid Will. Will Caveats are very fact-specific, and require a lot of investigating and gathering of evidence and witnesses. Because this makes litigating these issues expensive, Will Caveats are most often seen in large estates.
Sometimes a person makes or changes a Will because someone is manipulating that person, who is vulnerable for some reason, such as problems with memory or thinking, and/or dependency on the bad actor. This is called Undue Influence, and it can invalidate an otherwise valid Will or Codicil. These claims are often proved with circumstantial evidence, such as the person making or changing the Will is dependent upon the influencer, has been isolated, and the WIll disinherits family that one would expect to be in the Will.