A prenuptial agreement may seem like this mammoth contract, a legal steamroller that simply can't be stopped. This idea is born out of a common misunderstanding about prenups: that they are somehow immune to a legal challenge. In actuality, they are quite susceptible to a legal challenge if the circumstances warrant such a challenge.
One common reason why a prenuptial agreement is challenged and invalidated (in part or wholly) is for the silliest of reasons: the prenup was either never put in writing, or it was improperly executed. This is a reminder to anyone considering walking down the aisle that if you want a prenuptial agreement, you should have that discussion with your soon-to-be-spouse early on, and then draft it appropriately.
Another common theme with prenups that are successfully legally challenged is they contained improper information in some way. This could be due to false or incomplete information in the prenup, or it could be due to the prenup containing provisions that are illegal or improper.
It is also imperative for the spouses involved in the prenup to have enough time to consider the prenup, know the ramifications of signing it, and have enough time to read through the whole document. Failing these aspects, the prenup could be invalidated.
Last but not least, the prenup has to be "fair." This doesn't mean "equal." But it must at least be conscionable, meaning that if a judge were to review a prenup upon a legal challenge and determine that it is indeed too favorable to one spouse over the other, then he or she could deem it "unconscionable."
Source: FindLaw, "Top 10 Reasons a Premarital Agreement May be Invalid," Accessed April 13, 2018