Testamentary capacity of the deceased:
Dementia
Schizophrenia or paranoia

Undue influence or pressure:
Usually a family member or from people close to the deceased
Difficult to prove

The interpretation of the Will itself:
Ambiguous terms
Unclear intentions

Never leave a claim too late.
If an apparent valid Will is located and an executor applies for a grant of Probate it might be too late to complain about the validity of that Will afterwards.

If a grant of Probate has not been made then a caveat should be entered at the Probate Registry preventing a grant of any form being made. This will allow time for further investigations and negotiations to support the position of invalidity.

The Supreme Court takes into account the age, health, mental capacity, home surroundings and general environment in which the deceased was living to make an assessment as to whether the Will is valid or not.

Medical evidence is usually called from a doctor to confirm or qualify the mental capacity of the deceased and whether they could understand what they were reading and signing.