At Healey v Browne, after his wife`s death, a husband transferred property together to himself and his son. Although it was found that there was no mutual will (Donaldson QC accepted the contractual requirement), he considered that the second deceased was free to use the assets for his own economic interests until it was calculated to defeat the agreement: “If the trust obligation is breached by such a voluntary inter vivo provision of the assets concerned, the “crystallization” of the floating commitment must take place at the time of this provision.” (Note that Donaldson imposed a secret trust on QC in the circumstances that reduced the son`s interests to 50%, i.e. the interests held by the husband) If there is a warning to be sounded, it is that a competent lawyer must be maintained if the enforceable force of mutual will is an important consideration for a couple in their estate planning. The evidence of this lawyer in a subsequent court proceeding may be on the day of the execution or not of the mutual will. A common will is a single document, executed by more than one person (usually between spouses) who acts on his death with respect to the property of each signatory (unless he revokes the will during his life ([citation required) Although a single document, the common will is a separate distribution of the property by each executor (signatory) and will be considered as such upon admission to the estate. Mutual wills are two (or more) wills that are binding on both parties, so that the survivor, after the first death, is limited in his ability to surrender his property by the agreement he has made with the deceased. Historically, such wills have played an important role in ensuring that the property was transferred to the children of a marriage and not to the spouse of a widow or widower during a remarriage. Reciprocal wills are wills written by at least two people and signed according to an agreement between individuals that binds the survivors of them. Each agrees with the other not to change his will after the death of the other. The will of the mirror, made essentially on the same terms, is not considered a mutual will without the decisive agreement between individuals that the survivor is contractually bound by them not to change his will. Mutual will has four basic conditions and a strict standard of opposability: it is the agreement not to revoke the terms of the will that is essential to establishing mutual will. For aggrieved beneficiaries, it is often very difficult to see that such an agreement exists. The burden of proof is high because the will that has just passed away cannot say what they intended to do at the time of the formation of the will.
It has been found that a vague intention to leave everything to the children is not sufficient evidence of mutual will (Pridham/Pridham  SASC 204). While a more formal meeting of family members, during which the will and intentions of the founders of the will to establish reciprocal wills were explicitly discussed, (as well as other evidence of the process of making the will) may constitute sufficient evidence of a court to find mutual will (Hardy/Whitcome  NZHC 2382). The best evidence would be a formal contract to make wills.