Most people don't realise that anyone can challenge a Will in court if they feel that there has been any hint of collusion, or un due influence on the Testator (the person writing their Will).
To highlight what can happen take a look a this article from the Guardian. It also highlights what happens when someone dies intestate (i.e. dying without a valid Will).
In this case even though the deceased left over £10 million the second wife was only due the first £125,000 of the estate as the deceased will was contested and found to be invalid.
here is the link: High court declares will invalid