This article shows that the principle of legality of a tax norm is not exhausted in its 'source' component (i.e. that the norm is enacted by parliament) but also encompasses the 'content' element - a set of requirements for it to be deemed as good law. It is not just retroactivity of a law that is an issue, but legal certainty is also jeopardized by retroactive interpretation implying changes in interpretation of the same norm by authorities who understood it for a long period in a different sense. A threat to legal certainty also exists when a provision that was dormant for years is suddenly applied. A case study shows that the Serbian Parliament issued an authentic interpretation of a tax norm to assert essentially a different interpretation compared to the one well-established in the past, in order to solve a particular case, while a court used that interpretation to pass judgment in a pending case. Within the EU we find notable cases where not only are norms attributed a certain meaning from their inception, even though 60 years may have passed from their initial introduction to the possibility of the existence of a 'new meaning' being suggested for the first time, but taxpayers are made to suffer the consequences of the new interpretation. Had the legislator been able to pay more attention to the content of the bills and apply a more comprehensive approach to current issues, the legal certainty, in terms of avoiding 'innovative' retroactive interpretations, could be preserved.