Lack of Capacity
A trust or a will can be overturned on grounds of lack of legal capacity: that is, the person drafting the trust or will was either not mentally competent (i.e., under some mental illness at the time, i.e., alzheimers or dementia, or under the influence of some substance that rendered them incompetent.)
Undue Influence, Fraud or Duress
A trust or a will (or transaction involving a deceased) can also be invalidated on the grounds of undue influence. Where a person improperly "influences" another to give him or her a testamentary gift in a will or trust, that gift can be set aside if it was obtained through undue influence, fraud or duress. Undue influence can also be found where a decedent gives a testamentary gift to a caregiver, as well.
Improper Formalities &/or Forgery
First, a forged will or trust (or anything for that matter), is a nullity--it is invalid. An attested will (typed out) must be witnessed by two impartial witnesses. On the other hand, a holographic (handwritten) will is generally valid where the will is entirely in the decedent's handwriting, and signed by the decedent. Trusts, on the other hand, merely require a signature by the decedent, a property or cash to be given away, and identifiable beneficiaries. However, if any of those are lacking, the trust will be invalidated.