I came across a case from New Brunswick, In re Estate of Watts, 1933 CarswellNB 9 (S.C. App. Div.) in which the will-maker left to her husband the sum of $1 "as memento of the manner in which my husband treated me during our married life."
Her husband was not overly pleased with the will. After her death he challenged it in court. He argued that she was under an insane delusion that he had been unfaithful to her, and accordingly that she did not have the mental capacity to make the will. If the Court found that the will-maker was influenced in her decision to essentially disinherit her husband by an insane delusion about him, then she likely was incompetent and the will invalid. There was evidence that she did indeed believe that her husband was unfaithful.
But the Court had some trouble with the argument that the will-maker’s belief that her husband was unfaithful was a delusion, notably the evidence of the family physician that he had treated the husband for gonorrhea, and then treated her for the same disease.
The husband also argued that the amount of the gift was itself an indication of incapacity. In rejecting that argument, Gimmer J. for the court wrote at paragraph 12:
12 It has been held that mere estrangement, distrust, unfounded jealously and unjust resentment of fancied wrongs will not necessarily constitute delusions. They must be shown to be due to some erroneous belief for which there is no foundation in evidence. Mere peculiarities of mind and eccentricities of conduct in the testator are not in themselves sufficient to render him incompetent.