The courts, in determining whether a document represents a valid will, will look to a wide variety of evidence. This evidence could include whether the will had witness signatures, the document revoked previous wills, the document named an executor, the testator includes specific gifts and the handwriting was that of the testator. Other evidence may be helpful.
The further a document departs from the formal requirements of a will the harder it is for the court to find that the document can be found to be a will.
The best prevention against such a situation is to ensure you have a professionally prepared will by a lawyer.
However, if are in the situation where you are a potential beneficiary or executor of a defective will, you should immediately seek legal advice from a great law firm like Dyson Law.