We have a huge collection of stamps UMM (Unmounted Mind) & Used.
In 1974, errors & flaws in QE2 Decimal stamps came to light.
Since then until 1993 we have checked all the stamps that were issued for errors.
Some have been Catalogued but many many have not.
These newly discovered stamps in Vending & Counter booklets, definitives & commemorative sheets, Non Value Indicator (NVI) booklets and used stamps came to light.
Now these are on sale.
Many errors are singles only hence rare.
Errors in GB Stamps