I read in the Irish times a case about a man somewhere in Dublin that added such a clause to his will (some kind of codicil I think). He was always very friendly with a neighbour who used to look after him and drive him around etc. Anyhow, he left a majority of his estate to the neighbour and the remainder to family. He has one of these clauses. Anyhow, the family disputed it and ended up loosing the case and also the clause kicked in and they lost the original portion of the estate also.