It doesn't happen often, but the existence of fraud can cause a Will to be invalid. Fraud is simply a lie (called a misstatement of a material fact in legal parlance). Thus, if someone lies to a person creating a Will (called the testator) and that lie causes a different disposition under the Will than otherwise would have been made, then the Will is invalid for fraud.
Let's put this legal-talk to an actual example. A testator had two sons, Adam and Brian; Adam was an alcoholic, but rehabilitated himself and no longer drinks. The testator had previously disinherited Adam when he was an alcoholic, but then created a new Will giving Adam an equal share once he was rehabilitated. However, the testator warns Adam that "if I ever see you drink again, you’re out of my Will for good!"
A few weeks later, the testator and his other son Brian are walking down the street and they see someone exiting a bar with a drink in his hand that looks like Adam, but the testator isn't sure if it’s Adam. He asks Brian if that is his brother exiting the bar, and Brian says "that's my brother Adam all right" even though Brian can clearly tell that the man exiting the bar is not Adam. The testator then changes his Will and disinherits his son Adam based on that lie.
Since the Will in this example was created based on a lie, and the Will was changed in a way that would not have occurred absent the lie, the Will could, and very likely should, be overturned.
The problem, of course, is that these statements made to the testator must be established by credible evidence in court. And if no one else heard the lying statement by Brian and no one else knew exactly why the testator changed his Will, then there won't be any evidence to present in court—Brian certainly is not going to admit to the lie in open Court.
So fraud is a ground, albeit a difficult ground, that can be used to overturn a California Will or Trust (this same discussion applies to the creation or amendment to a Will or Trust too).