The vast majority of wills in California are presented to the probate court and their instructions are carried out without any problems. However, on rare occasions the validity of a will may be contested. Most successful challenges to a will are based on either the assertion that the person who made the will was not competent to do so, or that the testator was under the undue influence of someone else so that the will does not reflect the individual's true wishes.
Proving a will is not valid does not always end with the entire will being thrown out. A court has the discretion to invalidate the will in part and leave other parts of it enforceable, or to substitute provisions in a will.
If the challenge is to the competency of the testator, the executor has the burden of proving that the maker of the will was fully competent at the time the will was signed. On the other hand, if the will is challenged on the grounds that the maker was under the undue influence of another, the persons making the challenge have the burden of producing evidence that supports that claim.
The distribution of an estate according to the wishes of the individual is accomplished by writing a will, and the advice and assistance of an estate planning attorney is important when doing so. Since an objection to the will does not happen until after the individual's death, great care must taken in the writing and execution of the will and compliance with the required formalities. However, if a challenge is made to the will during probate, the executor will benefit from immediate legal assistance.
Source: California Law, "Probate Code Section 8250-8254," Nov. 1, 2014