It used to be that prenuptial agreements were considered ironclad documents that were nearly impossible to challenge. The documents would be signed before a couple walked down the aisle, and all the contents of the document would be appropriately enforced in case of a divorce. If that divorce were to happen, the spouses may not fully understand what is still contained in their prenup -- years may have passed since they last looked at it.
Thus, they may not agree with certain clauses or provisions in the prenup. They may present a legal challenge to those provisions, or the whole contract in general. Challenges to prenups can succeed, but it can be very difficult to achieve such a result. The challenging spouse needs to prove a variety of elements.
If the spouse says that the document was signed under duress, or that the document is fraudulent in some way, you need to consult an experienced family law attorney to help you understand the parameters of your case. Ultimately, you should consult an attorney anyway when you go through divorce -- but it is especially important when a prenup is involved, let alone a challenge to a prenup.
The spouse who enforces the prenup has a large burden of proof as well. They have to demonstrate that at the time of signing the prenup, all assets and liabilities were fully disclosed. In addition, there can be no coercive actions to make the spouse sign the prenup. The terms of the prenup must also be fair, and the deal must hold up over time, as it must be "conscionable" when the prenuptial agreement is enforced.
Source: Boston Herald, "Woman worries about terms of prenup," Gerald Nissenbaum, March 16, 2014