Although a lot of focus has been placed on "undue influence," some other grounds for challenging a Will include fraud, duress, menace, or mistake. The purpose of this post isn't to delve into particulars of each one but simply to make you aware of their existence. It's important to remember that under California Probate Code Section 6104, "[t]he execution or revocation of a will or a part of a will is ineffective to the extent the execution or revocation was procured by duress, menace, fraud, or undue influence."
Mistakes in Wills vs. Trusts
Mistakes are treated differently depending on whether they are present in a Will or Trust document.
When there's a substantial mistake of fact or law, a trust may be rescinded. On the other hand, if a Will as a whole is executed with testamentary intent, it can't be overturned on the basis of mistake.
In case you're not sure what a "mistake" is in the estate planning context, here are some examples. A mistake could be one where you didn't realize the effect of what was written in the Will or Trust. Alternatively, a mistake could be more technical in the sense that you didn't know the proper steps required to properly sign your estate planning documents. Still, another form of mistake might simply be a typo that has a substantial unintended effect on the distribution of your assets.
That being said, an experienced litigator in the trust and estates field may leverage one or more of these methods of attacking a Will or Trust, depending on the facts presented.