Yet the notice in the shops warning of the mistake is so small that customers may be completely unaware that their wills could be incorrect.
Do-it-yourself wills have been popular for many years. In theory they are simple and cheap. You can get one for a couple of pounds and all you do is fill in the gaps. You avoid a trip to the solicitor - and the legal costs.
The do-it-yourself wills come with explanatory notes. The current pack from WH Smith costs pounds 4.25. It includes comprehensive guidance notes on the formalities involved.
However, if you bought a will from the high street chain between October 1977 and March 1983 the notes had a serious flaw.
They omitted to tell you that the husband or wife of a beneficiary - a person who inherits under a will - should not be a witness to it. Otherwise, the gift is invalid and the spouse gets nothing.
WH Smith has issued a warning notice about the will forms - if you think your will may be wrong, make a new one and destroy the other. The shop will give you a new form free if you bring in the old one.
The retailer is at pains to point out that 'in no circumstances will our staff read your will'.
A spokesman said the notices had been in the shops for some years. I have been into my local branch in Surrey literally hundreds of times over the past 10 years and I had never seen it until this week.
You need the eyes of a hawk to spot it. The notice is very small, 15cm square. It was stuck on a glass cabinet with other notices including one about solvent abuse.
I was waiting in a queue right opposite it, but it was a chance in a million I spotted it. When I did the staff were extremely helpful. But I wonder how many people who have made wills during the relevant period have seen it.
A spokesman for WH Smith said: 'I have spoken to our Holborn Circus branch (in London).
'The manager has been there eight years and not one single person has brought one back.'
That could mean no one got his or her will wrong. On the other hand, perhaps no one spotted the warning.
The Smith will pack does have a disclaimer. No liability is accepted for any financial loss, personal distress or disappointment.
WH Smith says that the warning in the stores was not the result of anyone having a problem with their will. 'The omission was something we discovered ourselves when we were revamping them,' a spokesman said.
In theory, do-it-yourself wills look a tempting prospect. In practice, solicitors often joke that they make more money sorting out the problems left behind by home-made wills than the fees charged for drawing up a will in the first place.
For the sake of about pounds 50, the cost of a straightforward will, it is far better to let a solicitor do it for you.
This week the charity Age Concern has launched a postal will-writing service, operated by an independent solicitor and open to people of all ages. The cost is pounds 45 for a single will and pounds 65 for mirror wills - the same will for husband and wife.
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