Contesting a Will can be fraught with difficulty - not to mention emotion - given that an individual may likely be asserting a right to inherit against another family member. The ability to contest a Will depends very much on the circumstances of the case and the applicable law.
This becomes more complex once again when considered in an international context. There may be conflicting jurisprudence as to which laws apply, nevermind what those laws actually state. Often the grounds for varying a Will would relate to undue influence on the intestator by a beneficiary named in the Will though in European jurisdictions, a child of the deceased may have a right to a portion of the estate in a system known as forced heirs, and any failure to inherit the minimum portion assigned by law, could give rise to a right to contest the Will.