It isn't uncommon for a person to try to save money by writing a will themselves without the guidance of a qualified attorney. Recently, we helped a family probate one such will. The person typed up a will using a form they bought at an office supply company and later signed a handwritten codicil that changed all the distributions in the DIY will. The original will left some property to specific people and then left the residue to the person's heirs. Because the person didn't name an independent executor, the family had to pay for a determination of heirship so that the executor could determine who were the heirs in order to get permission to act like an independent executor. The alternative was a dependent executor, which requires court supervision of each step and court approval of all distributions.
The handwritten codicil was not valid because the person didn't hand write it themselves. In order for a document to be a valid holographic will or codicil, it must be wholly in the person's own handwriting among other requirements. If anyone else hand writes the document, it must be signed and witnessed in the same way any typewritten will would be in order to be valid, and notarized properly in order to be self-proved.
The person's attempt to save money by doing the will themselves cost the family an additional $3,500 over the normal cost of probate, over twice what it costs to have an attorney draft the will and powers of attorney.