Wills can be contested if you can prove undue influence.   In other words what they wanted was not what they signed.  Difficult to prove but it can be proven in some instances.  Assume the same would be true with a trust. 

Per the internet "  For a revocable living trust, the beneficiaries are all contingent upon the death of the grantor and lack the right to see the trust until the grantor passes. Once that occurs, the successor trustee takes control of the trust and its property listed in the document. The successor trustee must then provide a copy of the terms to all direct beneficiaries."