One of the more common reasons for litigation over an estate is when a new will is written or an existing will is changed late in life and the persons disadvantaged by the change allege that the testator (the person who made the will) was not mentally competent at the time of the change.
This is a hard assertion to substantiate, as is illustrated by a recent case. It involved a company director who owned a 1/3rd shareholding in a company. His two brothers owned the other 2/3rds of the shares. The man had intermittent mental problems and suffered from alcoholism.
Two years before he died, he altered his will to give his shares (which had previously been left to his two brothers) to his wife.
The court upheld the new will after it was challenged by the man’s brothers. They argued that he had not understood the value of the business and would not have changed his will had he done so. However, the fact that the man understood that he owned the shares was sufficient to defeat the challenge to the will: he did not have to have an idea of the value of the shares.