The perspective of anonymisation is one of ‘I don’t know who you are, but I know this about you’, while the perspective of anti-discrimination legislation is the complementary ‘I don’t know this about you, but I know who you are’. I look at how organisations have attempted to comply with the law, and show that this has led to confusion and lack of compliance. The fundamental problem arises from ambiguous and incompatible definitions, and recent changes to the law have made it worse. I illustrate some of the damaging adverse consequences, for both individuals and for society, that have arisen from this confusion.